drm/amd/amdgpu: get rid of else branch

else branch is pointless if it's right at the end of function and use
unlikely() on err path.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Nikola Pajkovsky <npajkovsky@suse.cz>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>