drm/i915: Gen3 HWSTAM is actually 32 bits

Bspec claims that HWSTAM is only 16 bits on gen3, but the other
interrupts registers are 32 bits and there are 18 valid interrupt
bits. Hence a 16 bit HWSTAM wouldn't be able to contain all the
bits, so it seems the spec is incorrect about the size of the
register. And indeed I can clear bits 16 and 17 just fine with
a 32 bit write. So let's adjust the code to treat the register
as 32 bits.

Acked-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170818183705.27850-14-ville.syrjala@linux.intel.com