drm: Clean up drm_calc_vbltimestamp_from_scanoutpos() vbl_status

Avoid confusion and don't use 'vbl_status' as both the
.get_scanout_position() return value and the return value from
drm_calc_vbltimestamp_from_scanoutpos().

While at it make 'vbl_status' unsigned and print it as hex in the
debug prints since it's a bitmask.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>