drm/i915/display: Implement fb_mmap callback function

If stolen memory allocation fails for fbdev, the driver will
fallback to system memory. Calculation of smem_start is wrong
for such framebuffer objs if the platform comes with no gmadr or
no aperture. Solve this by adding fb_mmap callback which will
use GTT if aperture is available otherwise will use cpu to access
the framebuffer.

v2: Use to_intel_fbdev() function(Jani)

Cc: Matthew Auld <matthew.auld@intel.com>
Cc: Andi Shyti <andi.shyti@linux.intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Imre Deak <imre.deak@intel.com>
Signed-off-by: Nirmoy Das <nirmoy.das@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Reviewed-by: Andrzej Hajda <andrzej.hajda@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230404143100.10452-5-nirmoy.das@intel.com

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

@@ -40,8 +40,10 @@
 #include <drm/drm_crtc.h>
 #include <drm/drm_fb_helper.h>
 #include <drm/drm_fourcc.h>
+#include <drm/drm_gem_framebuffer_helper.h>
 
 #include "gem/i915_gem_lmem.h"
+#include "gem/i915_gem_mman.h"
 
 #include "i915_drv.h"
 #include "intel_display_types.h"
@@ -119,6 +121,15 @@ static int intel_fbdev_pan_display(struct fb_var_screeninfo *var,
 	return ret;
 }
 
+static int intel_fbdev_mmap(struct fb_info *info, struct vm_area_struct *vma)
+{
+	struct intel_fbdev *fbdev = to_intel_fbdev(info->par);
+	struct drm_gem_object *bo = drm_gem_fb_get_obj(&fbdev->fb->base, 0);
+	struct drm_i915_gem_object *obj = to_intel_bo(bo);
+
+	return i915_gem_fb_mmap(obj, vma);
+}
+
 static const struct fb_ops intelfb_ops = {
 	.owner = THIS_MODULE,
 	DRM_FB_HELPER_DEFAULT_OPS,
@@ -130,6 +141,7 @@ static const struct fb_ops intelfb_ops = {
 	.fb_imageblit = drm_fb_helper_cfb_imageblit,
 	.fb_pan_display = intel_fbdev_pan_display,
 	.fb_blank = intel_fbdev_blank,
+	.fb_mmap = intel_fbdev_mmap,
 };
 
 static int intelfb_alloc(struct drm_fb_helper *helper,