drm/i915/pcode: rename sandybridge_pcode_* to snb_pcode_*

Prefer acronym-based naming to be in line with the rest of the driver.

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/20220112111740.1208374-1-jani.nikula@intel.com