agp/intel-gtt: Replace kernel.h with the necessary inclusions

When kernel.h is used in the headers it adds a lot into dependency hell,
especially when there are circular dependencies are involved.

Replace kernel.h inclusion with the list of what is really being used.
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/4d6a976459547407979f4b4c05a52785523e6bd8.1636977089.git.jani.nikula@intel.com

include/drm/intel-gtt.h

@@ -6,7 +6,10 @@
 
 #include <linux/agp_backend.h>
 #include <linux/intel-iommu.h>
-#include <linux/kernel.h>
+#include <linux/types.h>
+
+struct pci_dev;
+struct sg_table;
 
 void intel_gtt_get(u64 *gtt_total,
 		   phys_addr_t *mappable_base,