drm/xe/mmio: stop incorrectly triggering drm_warn

CI keeps triggering:

xe 0000:03:00.0: [drm] Restricting VRAM size to PCI resource size
(0x400000000->0x3fa000000)

Due to usable_size vs vram_size differences. However, we only want to
trigger the drm_warn() to let developers know that the system they are
using is going clamp the VRAM size to match the IO size, where they can
likely only use 256M of VRAM. Once we properly support small-bar we can
revisit this.

v2 (Lucas): Drop the TODO for now

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Gwan-gyeong Mun <gwan-gyeong.mun@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>