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 <ck.hu@mediatek.com>
Cc: Philipp Zabel <p.zabel@pengutronix.de>
Acked-by: Philipp Zabel <p.zabel@pengutronix.de>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161114090255.31595-4-daniel.vetter@ffwll.ch