drm/radeon: add helper rdev_to_drm(rdev)

Add helper rdev_to_drm(rdev), similar to amdgpu, most function should
access the "drm_device" with "rdev_to_drm(rdev)" instead, where amdgpu has
"adev_to_drm(adev)". It also makes changing from "*drm_device" to "drm_device"
in "radeon_devicce" later on easier.

Reviewed-by: Thomas Zimmermann <[e-mail]>
Tested-by: Thomas Zimmermann <[e-mail]>
Signed-off-by: Wu Hoi Pok <[e-mail]>
Signed-off-by: Alex Deucher <[e-mail]>

drivers/gpu/drm/radeon/radeon.h

···
 u32 cik_mm_rdoorbell(struct radeon_device *rdev, u32 index);
 void cik_mm_wdoorbell(struct radeon_device *rdev, u32 index, u32 v);
 
+static inline struct drm_device *rdev_to_drm(struct radeon_device *rdev)
+{
+	return rdev->ddev;
+}
+
 /*
  * Cast helper
  */