loader: move all gfx_fb.c stubs to common/gfx_fb_stub.c

All these files are the same, modulo one comment. Move them all into
common/gfx_fb_stub.c and adjust Makefiles accordingly.

Sponsored by: Netflix
Reviewed by: manu
Differential Revision: https://reviews.freebsd.org/D33428

(cherry picked from commit 91d462192db0dc33dbc4ca38f2b24d74c5ea2987)