drm/fb-helper: call vga_remove_vgacon automatically.

Add vga_remove_vgacon() call to
drm_fb_helper_remove_conflicting_pci_framebuffers().

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20190301092502.30948-3-kraxel@redhat.com