agp/intel-gtt: reduce intel-gtt dependencies more

Don't include stuff on behalf of users if they're not strictly necessary
for the header.

Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/7bcaa1684587b9b008d3c41468fb40e63c54fbc7.1636977089.git.jani.nikula@intel.com

drivers/char/agp/intel-gtt.c

@@ -20,6 +20,7 @@
 #include <linux/kernel.h>
 #include <linux/pagemap.h>
 #include <linux/agp_backend.h>
+#include <linux/intel-iommu.h>
 #include <linux/delay.h>
 #include <asm/smp.h>
 #include "agp.h"

drivers/gpu/drm/i915/gt/intel_ggtt.c

@@ -3,6 +3,7 @@
  * Copyright © 2020 Intel Corporation
  */
 
+#include <linux/agp_backend.h>
 #include <linux/stop_machine.h>
 
 #include <asm/set_memory.h>

include/drm/intel-gtt.h

@@ -4,10 +4,9 @@
 #ifndef _DRM_INTEL_GTT_H
 #define	_DRM_INTEL_GTT_H
 
-#include <linux/agp_backend.h>
-#include <linux/intel-iommu.h>
 #include <linux/types.h>
 
+struct agp_bridge_data;
 struct pci_dev;
 struct sg_table;