drm/i915/dpt: rename i915 specific functions to i915_dpt_ prefix
Follow the common convention of naming functions by file name, in this
case also clarifying which functions are i915 specific.
Reviewed-by: Juha-Pekka Heikkila <juhapekka.heikkila@gmail.com>
Link: https://patch.msgid.link/9bb94942ed4a15bcdd89be3f0029a2cb6cdf170d.1772030909.git.jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>