pxafb: introduce "struct pxafb_dma_buff" for palette and dma descriptors

Use structure and array for palette buffer and dma descriptors to:

1. better organize code for future expansion like overlays
2. separate palette and dma descriptors from frame buffer

Signed-off-by: eric miao <eric.miao@marvell.com>
Cc: "Antonino A. Daplas" <adaplas@pol.net>
Cc: Russell King <rmk@arm.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>