drm/i915/vblank: add and use intel_de_read64_2x32() to read vblank counter

Add intel_de_read64_2x32() wrapper for the uncore version of the same,
and use it to read the high and low frame registers. Avoid duplicating
code for existing helpers.

The slight functional difference is checking that the entire high
register remains the same across two reads, instead of just the part
we're interested in. This should be of no consequence. (Unless those
bits function as a PRNG.)

Cc: Ville Syrjälä <ville.syrjala@linux.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/33853549adff82045b95af527e14cfdff5712470.1673873708.git.jani.nikula@intel.com