vfb: Support vfb configuration from kernel cmdline

Add support to configure mode_option, framebuffer size and bpp
from kernel command line.

Change-Id: I2f08ed3851db4008569f1c77a195e46931674919
Signed-off-by: Shashank Mittal <mittals@codeaurora.org>
Signed-off-by: David Brown <davidb@codeaurora.org>
Signed-off-by: Bruce Levy <blevy@codeaurora.org>