drm/i915: Track media IP stepping separated from GT

Graphics and media IPs can have different stepping so a new field is
needed in intel_step_info.

The next patch will take care of rename gt_step to graphics_step.

Cc: Radhakrishna Sripada <radhakrishna.sripada@intel.com>
Cc: Matt Atwood <matthew.s.atwood@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211020002353.193893-2-jose.souza@intel.com