drm/i915: extract ibx_display_interrupt_update

This way all changes to SDEIMR all go through the same function, with
the exception of the (single-threaded) setup/teardown code.

For paranoia again add an assert_spin_locked.

v2: For even more paranoia also sprinkle a spinlock assert over
cpt_can_enable_serr_int since we need to have that one there, too.

v3: Fix the logic of interrupt enabling, add enable/disable macros for
the simple cases in the fifo code and add a comment. All requested by
Paulo.

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