fix xworld building with 24-bit lcd
also fix a couple unused vars

Change-Id: Ib05d2e22dd7477bc9bd193232bd3de8a0816beb2
Reviewed-on: http://gerrit.rockbox.org/1079
Reviewed-by: Michael Giacomelli <giac2000@hotmail.com>
Fixes xworld keymaps for ondavx747, hifimanhm60x, and cowond2

Change-Id: I62d7702d2e6541777f5553e27e166470004cd06d
Reviewed-on: http://gerrit.rockbox.org/1080
Reviewed-by: Michael Giacomelli <giac2000@hotmail.com>