drm/i915/vblank: use intel_de_read()

Use the intel_de_* functions for display registers.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Arun R Murthy <arun.r.murthy@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/ecfac5ebddcb46be1ddf2e34e52648fbf9fa4ca6.1673873708.git.jani.nikula@intel.com