powerpc/86xx: don't pretend that we support 8-bit pixels on the MPC8610 HPCD

If the video mode is set to 16-, 24-, or 32-bit pixels, then the pixel data
contains actual levels of red, blue, and green.  However, if the video mode
is set to 8-bit pixels, then the 8-bit value represents an index into color
table.  This is called "palette mode" on the Freescale DIU video controller.

The DIU driver does not currently support palette mode, but the MPC8610 HPCD
board file returned a non-zero (although incorrect) pixel format value for
8-bit mode.

Signed-off-by: Timur Tabi <timur@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>