drm/i915/dg1: Drop final use of IS_DG1_GRAPHICS_STEP

All production DG1 hardware has graphics stepping B0; there is no such
thing as C0.  As such, we can simplify

        IS_DG1_GRAPHICS_STEP(uncore->i915, STEP_A0, STEP_C0)

to just match DG1 in general.

Bspec: 44463
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230127224313.4042331-4-matthew.d.roper@intel.com