drm/xe: Simplify rebar sizing

"Right sizing" the PCI BAR is not necessary.  If rebar is needed
size to the maximum available.

Preserve the force_vram_bar_size sizing.

Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Michael J. Ruhl <michael.j.ruhl@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>