drm/msm/sde: handle vblank during modeset

During pre_modeset all the IRQs are disabled and set to invalid.
IRQs are enabled back on post_modeset. However, the userspace
vblank vote is left intact resulting in unbalanced vote.
Keep track of the vblank user vote during pre_modeset and
disable vblank, enable it back on post_modeset. No extra
synchronization is required as both the user requested vblank
and the modeset which is part of commit is executed in crtc
commit thread.

Change-Id: Ifa86cbea88683d0be8a01e2534373a6d4d648d58
Signed-off-by: Veera Sundaram Sankaran <veeras@codeaurora.org>