drm/xe: Nuke xe's copy of intel_fbdev_fb.h

For some reason xe and i915 each have an identical (fortunately)
copy of intel_fbdev_fb.h. The xe copy actually only gets included
by xe's intel_fbdev_fb.c, and the i915 copy by everyone else,
include intel_fbdev.c which is the actual caller of the
functions declared in the header.

This means the xe and i915 headers are free to define/declare
completely incompatible things and the build would still succeed
as long as the symbol names match.

That is not a good thing, so let's nuke xe's copy of the header
so that everyone will use the same header, and be forced to
agree on the same API/ABI.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240506183331.7720-1-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Acked-by: Lucas De Marchi <lucas.demarchi@intel.com>