drm: Pass 'flags' from the caller to .get_scanout_position()

Preparation for moving the early vblank IRQ logic into
radeon_get_crtc_scanoutpos().

v2: Fix radeon_drv.c compile warning (Mario)

Reviewed-by:  <mario.kleiner.de@gmail.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>