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

drivers/gpu/drm/i915/display/intel_dmc.c

@@ -31,6 +31,7 @@
 #include "intel_de.h"
 #include "intel_dmc.h"
 #include "intel_dmc_regs.h"
+#include "intel_step.h"
 
 /**
  * DOC: DMC Firmware Support

drivers/gpu/drm/xe/compat-i915-headers/i915_drv.h

@@ -18,10 +18,8 @@
 #include "xe_device.h"
 #include "xe_bo.h"
 #include "xe_pm.h"
-#include "xe_step.h"
 #include "i915_reg_defs.h"
 #include "i915_utils.h"
-#include "intel_step.h"
 #include "intel_runtime_pm.h"
 #include <linux/pm_runtime.h>