gma500: udelay(20000) it too long again

so replace it with mdelay(20).

Fixes build error:

  ERROR: "__bad_udelay" [drivers/staging/gma500/psb_gfx.ko] undefined!

Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>