MLK-11185 video: mxc ipuv3 fb: Support setting bpp to fbi->var from dtb

The value bpp of fbi->var can be initialized according to kernel
parameter video option.
But, for some cases, there is no video option.
In these cases, the bpp value should be configured according to the
'bpp' property in dtb if provided, otherwise the frame buffer will
always work in bpp 16 mode

Signed-off-by: Sandor Yu <R01008@freescale.com>