drm/i915/fbdev: Move intel_fb_initial_config() to fbdev helper

It is generic code and having it in the helper will let other drivers
benefit from it.

One change was necessary assuming this to be true:
INTEL_INFO(dev_priv)->num_pipes == dev->mode_config.num_crtc

Suggested-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: intel-gfx@lists.freedesktop.org
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190407165243.54043-4-noralf@tronnes.org