drm/i915: fix race when clearing RPS IIR bits

When disabling RPS interrupts there is a race where we disable RPS
inerrupts while the interrupt handler is running and the handler has
already latched the pending RPS interrupt from the master IIR register.
Afterwards the disabling path clears the PM IIR bits, making the state
of pending interrupts inconsistent from the interrupt handler's point of
view. This triggers the following warning: "The master control interrupt
lied (PM)!".

To fix this make sure that any running interrupt handler (which may
have already latched the master IIR) finishes before clearing the IIR
bits.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=87347


Signed-off-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>