drm/i915: Clarify irq_lock locking, special cases

Grab bag for all the special cases:
- i9xx_check_fifo_underruns is only called from crtc_enable hooks,
  i.e. process context.
- i915_enable_asle_pipestat is only called from interrupt postinstall
  hooks. So again process context.
- gen8_irq_power_well_post_enable is called from the runtime pm code,
  which again means process context.
- The open-coded hpd_irq_setup loop in _thaw is also running in process
  context.

So for all of them the plain _irq variant is sufficient.

Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>