drm/fb-helper: Remove drm_mode_config_fb.

Remove drm_mode_config_fb, I don't see the point of it.
To make it clear that it's ok to use plane->fb directly,
move up drm_drv_uses_atomic_modeset so the code is skipped for atomic
drivers that require plane_state->fb.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170703115106.18783-1-maarten.lankhorst@linux.intel.com


Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>