drm/i915: Rename GT_STEP to GRAPHICS_STEP

As now graphics and media can have different steppings this patch is
renaming all _GT_STEP macros to _GRAPHICS_STEP.

Future platforms will properly choose between _MEDIA_STEP and
_GRAPHICS_STEP for each new workaround.

Cc: Matt Atwood <matthew.s.atwood@intel.com>
Cc: Radhakrishna Sripada <radhakrishna.sripada@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Reviewed-by: Caz Yokoyama <caz.yokoyama@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211020002353.193893-3-jose.souza@intel.com