drm/i915: Fix DSI TE max_vblank_count handling

drm/i915: Fix DSI TE max_vblank_count handling

commit 33267703df15 ("drm/i915/dsi: Enable software vblank counter")
claims to get the mode_flags from the crtc_state, but in fact does
not. Fix it to do it right.

Cc: Vandita Kulkarni <vandita.kulkarni@intel.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210304170421.10901-5-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>