alpine: Default to 16bpp

24bpp support is going away, so since we can't do 32bpp and these cards
have basically no VRAM to begin with, drop to 16bpp.

Based on: https://cgit.freedesktop.org/xorg/driver/xf86-video-cirrus/patch/?id=c6f5096252923b1a344291e8045a15045e98dd27
--- a/src/alp_driver.c	2015-05-06 22:46:42.000000000 +0200
--- b/src/alp_driver.c	2015-05-06 22:46:42.000000000 +0200
@@ -555,7 +555,7 @@ AlpPreInit(ScrnInfoPtr pScrn, int flags)
 	 * The first thing we should figure out is the depth, bpp, etc.
 	 * We support both 24bpp and 32bpp layouts, so indicate that.
 	 */
-	if (!xf86SetDepthBpp(pScrn, 0, 0, 24, depth_flags)) {
+	if (!xf86SetDepthBpp(pScrn, 0, 0, 16, depth_flags)) {
 		return FALSE;
 	} else {
 		/* Check that the returned depth is one we support */