msm: mdss: add sysfs link to mdss_fb device

Add a symlink to mdss_fb device from fbdev node. This allows referring to
it more easily from userspace applications.

Change-Id: Ie0abacc6c376af89f62a5328afba2245ca082f7c
Signed-off-by: Adrian Salido-Moreno <adrianm@codeaurora.org>