Refactor custom video mode to allow testing

[joeladdison]

The custom video mode needs to be tested outside the firmware. The code relating to it should be refactored to allow this testing to happen, followed by tests being written and integrated into the build system.

[mithro]

@joeladdison What is the state of this? It looks like "make test-edid" is broken at https://github.com/timvideos/HDMI2USB-misoc-firmware/commits/nextgen

(H2U P=opsis T=hdmi2usb R=nextgen) tansell@tansell:~/foss/timvideos/hdmi2usb/HDMI2USB-merge$ make test-edid 
make -C test/edid check
make[1]: Entering directory `/usr/local/google/home/tansell/foss/timvideos/hdmi2usb/HDMI2USB-merge/test/edid'
cc -Wall -g -I/usr/local/google/home/tansell/foss/timvideos/hdmi2usb/HDMI2USB-merge/test/edid/../../firmware   -c -o edid_test.o edid_test.c
edid_test.c: In function ‘main’:
edid_test.c:42:9: error: too few arguments to function ‘generate_edid’
         generate_edid(edid, "OHW", "TV", 2015, "HDMI2USB-1", &mode);
         ^
In file included from edid_test.c:19:0:
/usr/local/google/home/tansell/foss/timvideos/hdmi2usb/HDMI2USB-merge/test/edid/../../firmware/edid.h:71:6: note: declared here
 void generate_edid(void *out,
      ^
edid_test.c:52:1: warning: control reaches end of non-void function [-Wreturn-type]
 };
 ^
make[1]: *** [edid_test.o] Error 1
make[1]: Leaving directory `/usr/local/google/home/tansell/foss/timvideos/hdmi2usb/HDMI2USB-merge/test/edid'
make: *** [test-edid] Error 2