drm/msm/mdp5: high precision vblank timestamp support

MDP5 has line count and frame count registers for each interface. Enable
these counters and use them to implement the get_vblank_timestamp drm
driver op.

The line counter starts with the value 1 at the beginning of the VSYNC
pulse and ends with value VTOTAL at the end of VFP. This value is used
to determine whether we're in blanking period or not, and an adjusted
value of this counter is used to get vpos as expected by
get_scanout_position. Since there is no way to calculate hpos, we always
set it to 0.

Signed-off-by: Archit Taneja <architt@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>