drm/fb_helper: Create a wrapper for unlink_framebuffer

Some drm drivers call unlink_framebuffer. Create a drm_fb_helper function
that wraps around these calls.

This is part of an effort to prevent drm drivers from calling fbdev
functions directly, in order to make fbdev emulation a top level drm
option.

v2:
- Added kerneldocs
- Added a check for non-NULL fb_helper before proceeding. This will
  make the helpers work when we have a module param for fbdev emulation

Signed-off-by: Archit Taneja <architt@codeaurora.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

include/drm/drm_fb_helper.h

@@ -145,6 +145,8 @@ void drm_fb_helper_fill_var(struct fb_info *info, struct drm_fb_helper *fb_helpe
 void drm_fb_helper_fill_fix(struct fb_info *info, uint32_t pitch,
 			    uint32_t depth);
 
+void drm_fb_helper_unlink_fbi(struct drm_fb_helper *fb_helper);
+
 int drm_fb_helper_setcmap(struct fb_cmap *cmap, struct fb_info *info);
 
 int drm_fb_helper_hotplug_event(struct drm_fb_helper *fb_helper);