drm/xe/slpc: Start SLPC before GuC submission on reset

The SLPC code has a strict 5ms timeout from when the start command is
queued to when we expect the reply to appear in memory. This works if
the CT channel is empty, but if the channel is busy there might be an
extra delay that causes the process to exceeded the timeout. We see
this issue when a reset occurs while userspace keeps submitting,
because the submission code is re-enabled first and it will start using
the channel to service those submissions.
To fix this, we can simply start SLPC before re-enabling submission.
This has also the benefit of not allowing submissions to go through with
an uninitialized SLPC.

Link: #375


Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Vinay Belgaumkar <vinay.belgaumkar@intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Vinay Belgaumkar <vinay.belgaumkar@intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Link: https://lore.kernel.org/r/20230628001642.3170070-1-daniele.ceraolospurio@intel.com