drm/i915: Speed up DMC firmware loading

drm/i915: Speed up DMC firmware loading

Currently we're doing:

1. acquire lock
2. write word to hardware
3. release lock
4. repeat from 1

to load the DMC firmware. Due to the cost of acquiring/releasing a lock,
and the size of the DMC firmware, this slows down DMC loading a lot.

This patch simply acquires the lock, writes the entire firmware,
then releases the lock.  Testing shows resume speedups
in the order of 10ms on platforms with DMC firmware (GEN9+).

v2: Per feedback from Chris & Ville there's no need to do the whole
    forcewake dance, so lose that bit (Chris, Ville)

v3: Actually send the new version of the patch...

v4: Don't acquire the uncore lock. Disable preempt. (Chris)

Signed-off-by: David Weinehall <david.weinehall@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170905131050.11655-1-david.weinehall@linux.intel.com
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>