drm/i915/lmem: Fail driver init if LMEM training failed

Boot firmware performs memory training and health assessment during
startup.  If the memory training fails, the firmware will consider the
GPU unusable and will instruct the punit to keep the GT powered down.
If this happens, our driver will be unable to communicate with the GT
(all GT registers will read back as 0, forcewake requests will timeout,
etc.) so we should abort driver initialization if this happens.  We can
confirm that LMEM was initialized successfully via sgunit register
GU_CNTL.

Bspec: 53111
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Cc: Caz Yokoyama <Caz.Yokoyama@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210420131842.164163-5-matthew.auld@intel.com