drm/radeon: Do not kfree() devres managed rdev

drm/radeon: Do not kfree() devres managed rdev

Since the allocation of the drivers main structure was changed to
devm_drm_dev_alloc() rdev is managed by devres and we shouldn't be calling
kfree() on it.

This fixes things exploding if the driver probe fails and devres cleans up
the rdev after we already free'd it.

Fixes: a9ed2f052c5c ("drm/radeon: change drm_dev_alloc to devm_drm_dev_alloc")
Signed-off-by: Daniel Palmer <daniel@0x0f.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>