drm/i915: split i915_driver_modeset_remove() to pre/post irq uninstall

Push irq uninstall further up, by splitting i915_driver_modeset_remove()
to two, the part with working irqs before irq uninstall, and the part
after irq uninstall. No functional changes.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200214135058.7580-2-jani.nikula@intel.com