drm/i915: split i915_driver_modeset_probe() to pre/post irq install

Pair the irq install and uninstall in the same layer. There are no
functional changes in the happy day scenario. The cleanup paths are
currently a mess though.

Note that modeset probe pre-irq + post-irq install are matched by
modeset driver remove pre-irq + post-irq uninstall, together, but not
independently. They are not symmetric pairs.

v2: don't add a new probe failure point here

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/20200219133756.13224-1-jani.nikula@intel.com