drm/xe/display: Create a dummy version for vga decode

This introduces an exclusive version of vga decode for xe.
Rest of the display changes will be re-used from i915.

Currently it adds just a dummy implementation. VGA decode
needs to be handled correctly in i915, proper implementation
will be adopted once the i915 changes are finalized and merged
in upstream.

v2: Addressed Arun's review comments

Signed-off-by: Uma Shankar <[e-mail]>
Reviewed-by: Arun R Murthy <[e-mail]>
Acked-by: Jani Nikula <[e-mail]>
Signed-off-by: Rodrigo Vivi <[e-mail]>

drivers/gpu/drm/xe/Makefile

···
 	display/xe_hdcp_gsc.o \
 	display/xe_plane_initial.o \
 	display/xe_display_rps.o \
+	display/xe_display_misc.o \
 	display/intel_fbdev_fb.o \
 	display/intel_fb_bo.o \
 	display/ext/i915_irq.o \

drivers/gpu/drm/xe/display/xe_display_misc.c

···
+// SPDX-License-Identifier: MIT
+/*
+ * Copyright © 2023 Intel Corporation
+ */
+
+#include "intel_display_types.h"
+
+struct pci_dev;
+
+unsigned int intel_gmch_vga_set_decode(struct pci_dev *pdev, bool enable_decode);
+
+unsigned int intel_gmch_vga_set_decode(struct pci_dev *pdev, bool enable_decode)
+{
+	/* ToDo: Implement the actual handling of vga decode */
+	return 0;
+}