drm/i915: fix locking around ironlake_enable|disable_display_irq

drm/i915: fix locking around ironlake_enable|disable_display_irq

The haswell unclaimed register handling code forgot to take the
spinlock. Since this is in the context of the non-rentrant interupt
handler and we only have one interrupt handler it is sufficient to
just grab the spinlock - we do not need to exclude any other
interrupts from running on the same cpu.

To prevent such gaffles in the future sprinkle assert_spin_locked over
these functions. Unfornately this requires us to hold the spinlock in
the ironlake postinstall hook where it is not strictly required:
Currently that is run in single-threaded context and with userspace
exlcuded from running concurrent ioctls. Add a comment explaining
this.

v2: ivb_can_enable_err_int also needs to be protected by the spinlock.
To ensure this won't happen in the future again also sprinkle a
spinlock assert in there.

v3: Kill the 2nd call to ivb_can_enable_err_int I've accidentally left
behind, spotted by Paulo.

Cc: Paulo Zanoni <przanoni@gmail.com>
Reviewed-by: Paulo Zanoni <przanoni@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>