drm/i915/display: include intel_step.h where needed

Include what you use. With this, we can drop the include along with
xe_step.h from xe compat i915_drv.h.

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Acked-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/abee71a6c6edbd1a3ddf0f97838977e53feaa5ff.1717004739.git.jani.nikula@intel.com