drm/irq: Unexport drm_vblank_count

drm/irq: Unexport drm_vblank_count

No one outside of drm_irq.c should ever need this. The correct way to
implement get_vblank_count for hw lacking a vblank counter is
drm_vblank_no_hw_counter. Fix this up in mtk, which is the only
offender left over.

Cc: CK Hu <[email protected]>
Cc: Philipp Zabel <[email protected]>
Acked-by: Philipp Zabel <[email protected]>
Reviewed-by: Alex Deucher <[email protected]>
Signed-off-by: Daniel Vetter <[email protected]>
Link: http://patchwork.freedesktop.org/patch/msgid/[email protected]