tridentfb: preserve memory type settings

Do not overwrite bits which contain memory type settings.  It removes
noise pixels ("snow") on Blade3D and 3DImage chips.
Signed-off-by: NKrzysztof Helt <krzysztof.h1@wp.pl>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

drivers/video/tridentfb.c

@@ -1095,7 +1095,10 @@ static int tridentfb_set_par(struct fb_info *info)
 	vga_mm_wseq(par->io_virt, 4, 0x0E); /* memory mode enable bitmaps ?? */
 
 	/* divide clock by 2 if 32bpp chain4 mode display and CPU path */
-	write3CE(par, MiscExtFunc, (bpp == 32) ? 0x1A : 0x12);
+	tmp = read3CE(par, MiscExtFunc) & 0xF0;
+	if (bpp == 32)
+		tmp |= 8;
+	write3CE(par, MiscExtFunc, tmp | 0x12);
 	write3CE(par, 0x5, 0x40);	/* no CGA compat, allow 256 col */
 	write3CE(par, 0x6, 0x05);	/* graphics mode */
 	write3CE(par, 0x7, 0x0F);	/* planes? */