drm: xlnx: dpsub: Fix DMADEVICES Kconfig dependency

The dpsub driver uses the DMA engine API, and thus selects DMA_ENGINE to
provide that API. DMA_ENGINE depends on DMADEVICES, which can be
deselected by the user, creating a possibly unmet indirect dependency:

WARNING: unmet direct dependencies detected for DMA_ENGINE
  Depends on [n]: DMADEVICES [=n]
  Selected by [m]:
  - DRM_ZYNQMP_DPSUB [=m] && HAS_IOMEM [=y] && (ARCH_ZYNQMP || COMPILE_TEST [=y]) && COMMON_CLK [=y] && DRM [=m] && OF [=y]

Add a dependency on DMADEVICES to fix this.
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org>

drivers/gpu/drm/xlnx/Kconfig

@@ -2,6 +2,7 @@ config DRM_ZYNQMP_DPSUB
 	tristate "ZynqMP DisplayPort Controller Driver"
 	depends on ARCH_ZYNQMP || COMPILE_TEST
 	depends on COMMON_CLK && DRM && OF
+	depends on DMADEVICES
 	select DMA_ENGINE
 	select DRM_GEM_CMA_HELPER
 	select DRM_KMS_CMA_HELPER