drm/radeon: change drm_dev_alloc to devm_drm_dev_alloc

"drm_dev_alloc" is deprecated, in order to use the newer "devm_drm_dev_alloc",
the "drm_device" is stored inside "radeon_device", by changing "rdev_to_drm(rdev)"
other functions still gain access to the member "drm_device". Also, "devm_drm_dev_alloc"
is now allocating "radeon_device", allocation inside "radeon_driver_load_kms" has to be
removed.

In "radeon_device_init", it originally assigned "rdev->dev" etc. However it is already
done right after "devm_drm_dev_alloc" as you can see down below. It is better remove them.

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>