drm/xe/rtp: Rename STEP to GRAPHICS_STEP

drm/xe/rtp: Rename STEP to GRAPHICS_STEP

Rename the RTP match in order to prepare the code base to check for the
media version. Up until MTL, the graphics vs media distinction wrt to
stepping was not ver relevant as they were the same GT. However, with
MTL this is no longer true.

Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://lore.kernel.org/r/20230526164358.86393-9-lucas.demarchi@intel.com
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>