drm/i915/skl: s/SKL/SKYLAKE for platform/subplatform defines

drm/i915/skl: s/SKL/SKYLAKE for platform/subplatform defines

Follow consistent naming convention. Replace SKL with
SKYLAKE and Replace IS_SKL_GRAPHICS_STEP with
IS_SKYLAKE() && IS_GRAPHICS_STEP().

v2:
- Change subject skl instead of SKL(Anusha)

v3:
- Unrolled wrapper IS_SKL_GRAPHICS_STEP.
- Replace with IS_PLATFORM && DISPLAY_STEP(tvrtko/jani)

v4:
- Removed the unused macro.

Cc: Anusha Srivatsa <anusha.srivatsa@intel.com>
Signed-off-by: Dnyaneshwar Bhadane <dnyaneshwar.bhadane@intel.com>
Reviewed-by: Anusha Srivatsa <anusha.srivatsa@intel.com>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Radhakrishna Sripada <radhakrishna.sripada@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230801135344.3797924-4-dnyaneshwar.bhadane@intel.com