drm/amdgpu: Stop using the DRIVER debugging flag for vblank debugging messages

These are some very loud debug statements that get printed on every
vblank when driver level debug printing is enabled in DRM, and doesn't
really tell us anything that isn't related to vblanks. So let's move
this over to the proper debug flag to be a little less spammy with our
debug output.

Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>