drivers/video/am335x-fb: Properly point framebuffer behind palette

drivers/video/am335x-fb: Properly point framebuffer behind palette

The DMA was outputting the palette on the screen because the base
for the DMA was not after the palette. In addition to that, the ceiling was
also too high, this led that the output on the screen was shifted.

NOTE: According to the TRM, even in 16/24bit mode a palette is required
in the first 32 bytes of the framebuffer.

See also:
https://e2e.ti.com/support/arm/sitara_arm/f/791/p/234967/834483#834483

"In this mode, the LCDC will assume all information is data and thus you
need to ensure that the DMA points to the first pixel of data and not the
first entry in the frame buffer which is the beginning of the 512 byte
palette."

Signed-off-by: Martin Pietryka <martin.pietryka@chello.at>
Reviewed-by: Hannes Schmelzer <oe5hpm@oevsv.at>
Tested-by: Hannes Schmelzer <oe5hpm@oevsv.at>