drm/amdgpu: call pci_[un]register_driver() directly

Rather than calling the deprecated drm_pci_init() and
drm_pci_exit() which just wrapped the pci functions
anyway.

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