drm/i915: switch to drm_fb_helper_remove_conflicting_pci_framebuffers

No need for a home-grown version, the generic helper should work just
fine.  It also handles vgacon removal these days, see commit
1c74ca7a

 ("drm/fb-helper: call vga_remove_vgacon automatically."),
so that can be removed too.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20190822090645.25410-4-kraxel@redhat.com