video: fbdev: fix possible null dereference

we were dereferencing edid first and the NULL check was after
accessing that. now we are using edid only if we know that
it is not NULL.

Signed-off-by: Sudip Mukherjee <sudip@vectorindia.org>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>