drm/irq: Unexport drm_vblank_on/off

Only remaining use was in amdgpu, and trivial to convert over to
drm_crtc_vblank_* variants.

Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Christian König <christian.koenig@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161114090255.31595-5-daniel.vetter@ffwll.ch