Reduce the number of BRDSrcs run in DM

Testing to e.g. Index8 is useful, but not usefully different across
all sampleSizes and Modes. (Same for Gray.) Drop several Srcs.

BUG=skia:5307
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1990173003

Review-Url: https://codereview.chromium.org/1990173003
1 file changed