drm/xe: Map the entire BAR0 and hold onto the initial mapping

Both MMIO registers and GGTT for root tile will need to be used earlier
during probe. Don't rely on tile count to compute the mapping size.
Further more, there's no need to remap after figuring out the real
resource size.

Signed-off-by: Michał Winiarski <michal.winiarski@intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>