drm/i915/guc: (re)initialise doorbell h/w when enabling GuC submission

During a hibernate/resume cycle, the whole system is reset, including
the GuC and the doorbell hardware. Then the system is booted up, drivers
are loaded, etc -- the GuC firmware may be loaded and set running at
this point. But then, the booted kernel is replaced by the hibernated
image, and this resumed kernel will also try to reload the GuC firmware
(which will fail). To recover, we reset the GuC and try again (which
should work). But this GuC reset doesn't also reset the doorbell
hardware, so it can be left in a state inconsistent with that assumed
by the driver and/or the newly-loaded GuC firmware.

It would be better if the GuC reset also cleared all doorbell state,
but that's not how the hardware currently works; also, the driver cannot
directly reprogram the doorbell hardware (only the GuC can do that).

So this patch cycles through all doorbells, assigning and releasing each
in turn, so that all the doorbell hardware is left in a consistent
state, no matter how it was programmed by the previously-running kernel
and/or GuC firmware.

v2: don't use kmap_atomic() now that client page 0 is kept mapped.

Signed-off-by: Dave Gordon <david.s.gordon@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1465837054-16245-2-git-send-email-david.s.gordon@intel.com