drm/radeon: pass drm dev radeon_agp_head_init directly

Pass drm dev directly as rdev->ddev gets initialized later on
at radeon_device_init().

Bug: https://bugzilla.kernel.org/show_bug.cgi?id=214375


Signed-off-by: Nirmoy Das <nirmoy.das@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org