drm/xe/mmio: Map MMIO BAR using managed version of pci_iomap

drm/xe/mmio: Map MMIO BAR using managed version of pci_iomap

This will allow us to simplify our custom release action where we
will keep only zeroing of the xe->mmio.regs as we still rely on it
all checks during all xe_mmio operations. While around, add missing
kernel-doc for the function and update the error message.

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patch.msgid.link/20260622132342.19600-3-michal.wajdeczko@intel.com