drm/i915: Reduce locking in gen8 IRQ handler

drm/i915: Reduce locking in gen8 IRQ handler

Similar in vain in reducing the number of unrequired spinlocks used for
execlist command submission (where the forcewake is required but
manually controlled), we know that the IRQ registers are outside of the
powerwell and so we can access them directly. Since we now have direct
access exported via I915_READ_FW/I915_WRITE_FW, lets put those to use in
the irq handlers as well.

In the process, reorder the execlist submission to happen as early as
possible.

v2: Restrict the untraced register mmio to just the GT path (i.e. the
hotpath for execlists)

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>