drm/amdgpu: Remove dead static variable

The static struct drm_driver *driver was
not used because drm_pci_init was deprecated

v2: Remove static pointer pdriver(Christian)

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