drm/xe/device: Convert register access to use xe_mmio

drm/xe/device: Convert register access to use xe_mmio

Stop using GT pointers for register access.  Since a GT was passed as a
parameter to verify_lmem_ready() solely as a way to do MMIO accesses,
change the parameter to xe_device, which more accurately reflects that
this is a device-wide operation.

v2:
 - Expand commit message to explain why verify_lmem_ready()'s parameter
   changes.  (Rodrigo)

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240910234719.3335472-62-matthew.d.roper@intel.com