drm/i915/fbdev: Add intel_fbdev_get_map()

The vaddr of the fbdev framebuffer is private to the struct
intel_fbdev, so this function is needed to access it for drm_panic.
Also the struct i915_vma is different between i915 and xe, so it
requires a few functions to access fbdev->vma->iomap.

Signed-off-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://lore.kernel.org/r/20250624091501.257661-3-jfalempe@redhat.com


Signed-off-by: Maarten Lankhorst <dev@lankhorst.se>