drm/omap: fix pitch round-up

At the moment we calculate the buffer's pitch with:

  pitch = width * DIV_ROUND_UP(bpp, 8)

For CLUT modes with bpp of 1/2/4/8 this gives wrong result, and the
correct pitch is:

  pitch = DIV_ROUND_UP(width * bpp, 8)

In practice this doesn't change anything, as we don't support CLUT
modes, but it's better to have the pitch calculation correct.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>