pvr2fb: fix pseudo_palette array overrun and typecast

- the pseudo_palette has only 16 elements. Do not write if regno (the array
  index) is more than 15.
- if using generic drawing libraries, the typecast of pseudo_palette is
  always u32 *

Signed-off-by: Antonino Daplas <adaplas@gmail.com>
Acked-by: Paul Mundt <lethal@linux-sh.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>