powerpc: remove support for bootmem-allocated memory for the DIU driver

Early versions of the Freescale DIU framebuffer driver depended on a bootmem
allocation of memory for the video buffer.  The need for this feature was
removed in commit 6b51d51a, so now we can remove the platform-specific code
that allocated that memory.

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