drm/i915: Clarify flow for disabling IRQs on storms

This is rather confusing to look at as-is:
dev_priv->display.hpd_irq_setup(dev_priv); in intel_hpd_irq_handler()
handles disabling the actual HPD IRQ, while
intel_hpd_irq_storm_disable() handles moving the HPD pin state over from
MARK_DISABLED to DISABLED along with enabling polling for it.

Changes since v3:
- Rename i915_hpd_irq_storm_disable() to
  i915_hpd_irq_storm_switch_to_polling() - Rodrigo Vivi

Signed-off-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181106213017.14563-5-lyude@redhat.com