drm/i915/dsi: Enable software vblank counter

In case of DSI cmd mode, we get hw vblank counter updated after the TE
comes in, if we try to read the hw vblank counter in te handler we
wouldnt have the udpated vblank counter yet. This will lead to a state
where we would send the vblank event to the user space in the next te,
though the frame update would have completed in the first TE duration
itself. Hence switch to using software timestamp based vblank counter.

v2: Use mode_flags from crtc_state (Ville)

Acked-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Vandita Kulkarni <vandita.kulkarni@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200924124209.17916-6-vandita.kulkarni@intel.com