drm/i915/dsc: have intel_dsc_pps_read_and_verify() return the value

drm/i915/dsc: have intel_dsc_pps_read_and_verify() return the value

Register read functions usually return the value instead of passing via
pointer parameters. The calling code becomes easier to read.

Make the name conform to existing style better while at it.

Cc: Suraj Kandpal <suraj.kandpal@intel.com>
Cc: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Suraj Kandpal <suraj.kandpal@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/89b7d70bb19114ab3ff0e150a4b862d8032f136d.1693933849.git.jani.nikula@intel.com