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 <tzimmermann@suse.de>
Tested-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Wu Hoi Pok <wuhoipok@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>