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

Split inte_modeset_init() to parts before and after irq install, to
facilitate further cleanup. The error paths are a mess, otherwise 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/20200224120828.22105-1-jani.nikula@intel.com