drm/aperture: Inline fbdev conflict helpers into aperture helpers

Fbdev's helpers for handling conflicting framebuffer drivers are
related to framebuffer apertures, not console emulation. Therefore
remove them from drm_fb_helper.h and inline them into the aperture
helpers. No functional changes.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210412131043.5787-4-tzimmermann@suse.de