drm/i915: don't save/restore irq regs for kms

We already call drm_irq_install/uninstall at the right time, which
will set up the irq registers with the correct values (through the
preinstall hooks).

For kms this is at best harmless, in the worst case we get an
interrupt when we don't really expect it.

v2: Fixup the logic, noticed by Paulo Zanoni.

Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Tested-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>