msm_fb: lvds: Fix uninitialized variable warnings

The compiler doesn't know that there are only two values for
mfd->panel_info.bpp. Add an else arm to catch the unknown third
value as a BUG() so that we avoid writing initialized variables
to registers.

Change-Id: I643df7e256cda59a4af73571e1cc5eef610d781f
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>