hw/display/ramfb: Compile the ramfb code only when CONFIG_FW_CFG_DMA is set

According to the ramfb_setup() function, the ramfb device needs fw_cfg
with DMA, so we should also only compile and link it into those targets
which support it, to avoid that the device shows up on systems where it
can not be used at all (e.g. s390x).

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-id: 1534786083-26559-1-git-send-email-thuth@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

hw/display/Makefile.objs

-common-obj-y += ramfb.o
-common-obj-y += ramfb-standalone.o
+common-obj-$(CONFIG_FW_CFG_DMA) += ramfb.o
+common-obj-$(CONFIG_FW_CFG_DMA) += ramfb-standalone.o
 
 common-obj-$(CONFIG_ADS7846) += ads7846.o
 common-obj-$(CONFIG_VGA_CIRRUS) += cirrus_vga.o