platinumfb: fix misplaced parenthesis

Since `+' has a higher precedence than the trinary operator `?', this
added `hres * (1 << color_mode)' to the boolean testing videomode and
depth.

Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Ville Syrjala <syrjala@sci.fi>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>