drm/i915/de: s/intel_de_read64_2x32()/intel_de_read64_2x32_volatile()/

drm/i915/de: s/intel_de_read64_2x32()/intel_de_read64_2x32_volatile()/

The raison d'etre of intel_de_read64_2x32() is that it can
handle registers where volatile values are split across two
registers. I don't like that it's being used needlessly.

Rename it to intel_de_read64_2x32_volatile() to make it
more clear when it should be used.

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patch.msgid.link/20260331154259.24600-9-ville.syrjala@linux.intel.com