drm/i915/fbdev: Enable late fbdev initial configuration

If the initial fbdev configuration (intel_fbdev_initial_config()) runs
and there still no sink connected it will cause
drm_fb_helper_initial_config() to return 0 as no error happened (but
internally the return is -EAGAIN).  Because no framebuffer was
allocated, when a sink is connected intel_fbdev_output_poll_changed()
will not execute drm_fb_helper_hotplug_event() that would trigger
another try to do the initial fbdev configuration.

So here allowing drm_fb_helper_hotplug_event() to be executed when there
is no framebuffer allocated and fbdev was not set up yet.

This issue also happens when a MST DP sink is connected since boot, as
the MST topology is discovered in parallel if
intel_fbdev_initial_config() is executed before the first sink MST is
discovered it will cause this same issue.

This is a follow-up patch of
https://patchwork.freedesktop.org/patch/196089/

Changes from v1:
- not creating a dump framebuffer anymore, instead just allowing
  drm_fb_helper_hotplug_event() to execute when fbdev is not setup yet.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=104158
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=104425
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: stable@vger.kernel.org # v4.15+
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Tested-by: Paul Menzel <pmenzel@molgen.mpg.de>
Tested-by: frederik <frederik.schwan@linux.com> # 4.15.17
Tested-by: Ian Pilcher <arequipeno@gmail.com>
Acked-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180418234158.9388-1-jose.souza@intel.com