drivers: gpu: drm: virtio: fix dependency of DRM_VIRTIO_GPU on VIRTIO

VIRTIO itself has no dependencies and therefore can easily be just
select'ed, instead of depending on it. The current depends on causes
any others trying to select VIRTIO to fail like this:

   drivers/gpu/drm/Kconfig:74:error: recursive dependency detected!
   drivers/gpu/drm/Kconfig:74:	symbol DRM_KMS_HELPER is selected by DRM_VIRTIO_GPU
   drivers/gpu/drm/virtio/Kconfig:2:	symbol DRM_VIRTIO_GPU depends on VIRTIO
   drivers/virtio/Kconfig:2:	symbol VIRTIO is selected by GPIO_VIRTIO
   drivers/gpio/Kconfig:1618:	symbol GPIO_VIRTIO depends on GPIOLIB
   drivers/gpio/Kconfig:14:	symbol GPIOLIB is selected by I2C_MUX_LTC4306
   drivers/i2c/muxes/Kconfig:47:	symbol I2C_MUX_LTC4306 depends on I2C
   drivers/i2c/Kconfig:8:	symbol I2C is selected by FB_DDC
   drivers/video/fbdev/Kconfig:63:	symbol FB_DDC depends on FB
   drivers/video/fbdev/Kconfig:12:	symbol FB is selected by DRM_KMS_FB_HELPER
   drivers/gpu/drm/Kconfig:80:	symbol DRM_KMS_FB_HELPER depends on DRM_KMS_HELPER

It seems that having both 'depends on' as well as 'select' on the same symbol
sends us into big trouble, and Kconfig can't break up the circular dependency
(note that in the tested configuration, neither I2C, FB or DRM are enabled at
all). Perhaps we could consider this a bug in Kconfig, but the trouble can
easily be circumvented by changing 'depends on' into 'select'.

DRM_VIRTIO_GPU also depends on VIRTIO_MENU, so even after this change, that
option will only show up if the user already enabled virtio in the config.

This change didn't cause any changes in the .config after menuconfig run,
so we should be completely safe here.
Signed-off-by: Enrico Weigelt, metux IT consult <info@metux.net>
Reviewed-by: Anthoine Bourgeois <anthoine.bourgeois@gmail.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20201204131221.2827-1-info@metux.netSigned-off-by: Gerd Hoffmann <kraxel@redhat.com>

drivers/gpu/drm/virtio/Kconfig

 # SPDX-License-Identifier: GPL-2.0-only
 config DRM_VIRTIO_GPU
 	tristate "Virtio GPU driver"
-	depends on DRM && VIRTIO && VIRTIO_MENU && MMU
+	depends on DRM && VIRTIO_MENU && MMU
+	select VIRTIO
 	select DRM_KMS_HELPER
 	select DRM_GEM_SHMEM_HELPER
 	select VIRTIO_DMA_SHARED_BUFFER