drm/i915: abstract intel_mode_config_init() from intel_modeset_init()

The i915 specific mode config init code is too specific and detailed to
have open in a high level function. Abstract away. No functional
changes.

v2: nest drm_mode_config_init() in the function too (Chris)

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190920185421.17822-5-jani.nikula@intel.com