drm/i915/gt: Add dedicated MCR lock

We've been overloading uncore->lock to protect access to the MCR
steering register.  That's not really what uncore->lock is intended for,
and it would be better if we didn't need to hold such a high-traffic
spinlock for the whole sequence of (apply steering, access MCR register,
restore steering).  Let's create a dedicated MCR lock to protect the
steering control register over this critical section and stop relying on
the high-traffic uncore->lock.

For now the new lock is a software lock.  However some platforms (MTL
and beyond) have a hardware-provided locking mechanism that can be used
to serialize not only software accesses, but also hardware/firmware
accesses as well; support for that hardware level lock will be added in
a future patch.

v2:
 - Use irqsave/irqrestore spinlock calls; platforms using execlist
   submission rather than GuC submission can perform MCR accesses in
   interrupt context because reset -> errordump happens in a tasklet.

Cc: Chris Wilson <chris.p.wilson@linux.intel.com>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Balasubramani Vivekanandan <balasubramani.vivekanandan@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Balasubramani Vivekanandan <balasubramani.vivekanandan@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221128233014.4000136-4-matthew.d.roper@intel.com

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