fbdev: export fb_edid_add_monspecs() for modules, improve algorithm

fb_edid_add_monspecs() should also be exported for use in modules, and it
requires a dummy version for the case, when CONFIG_FB_MODE_HELPERS is not
selected. This patch also improves the algorithm by removing a redundant
memory allocation, adds function documentation, adds data verification and
replaces memmove() with memcpy().

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>