drm/meson: remove firmware framebuffers

In case we are using simplefb or another conflicting framebuffer, make
the call to drm_fb_helper_remove_conflicting_framebuffers()

Signed-off-by: Maxime Jourdan <mjourdan@baylibre.com>
Acked-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181210092853.13050-1-mjourdan@baylibre.com