Fixes the following W=1 kernel build warning(s):

 drivers/gpu/drm/radeon/radeon_irq_kms.c:53:13: warning: no previous prototype for ‘radeon_driver_irq_handler_kms’ [-Wmissing-prototypes]
 53 | irqreturn_t radeon_driver_irq_handler_kms(int irq, void *arg)
 | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 drivers/gpu/drm/radeon/radeon_irq_kms.c:120:6: warning: no previous prototype for ‘radeon_driver_irq_preinstall_kms’ [-Wmissing-prototypes]
 120 | void radeon_driver_irq_preinstall_kms(struct drm_device *dev)
 | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 drivers/gpu/drm/radeon/radeon_irq_kms.c:152:5: warning: no previous prototype for ‘radeon_driver_irq_postinstall_kms’ [-Wmissing-prototypes]
 152 | int radeon_driver_irq_postinstall_kms(struct drm_device *dev)
 | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 drivers/gpu/drm/radeon/radeon_irq_kms.c:171:6: warning: no previous prototype for ‘radeon_driver_irq_uninstall_kms’ [-Wmissing-prototypes]
 171 | void radeon_driver_irq_uninstall_kms(struct drm_device *dev)
 | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: "Christian König" <christian.koenig@amd.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Gareth Hughes <gareth@valinux.com>
Cc: amd-gfx@lists.freedesktop.org
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>