drm/i915: optimize ilk/snb irq handler

We only need to read/write the south interrupt register if the
corresponding bit is set in the north master interrupt register.
Noticed while reading our interrupt handling code.

Same optimization has already been applied on ivb in

commit 0e43406b
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Wed May 9 21:45:44 2012 +0100

    drm/i915: Simplify interrupt processing for IvyBridge

    We can take advantage that the PCH_IIR is a subordinate register to
    reduce one of the required IIR reads, and that we only need to clear
    interrupts handled to reduce the writes. And by simply tidying the code
    we can reduce the line count and hopefully make it more readable.

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