va: decoders pass rate in fluster

I ran fluster comparing va plugin in gst-plugins-bad vs gstreamer-vaapi

These are the results (iHD):

	VA	VAAPI
H264	128/135	126/135
H265	120/147	142/147
VP9	150/303	216/303
VP8	55/61	57/61

Checking the VP9 results I realize that it can be improved disabling image formats. It seems that CSC for decoders is not 100% correct. Something that @He_Junyan told me, but I took it as a compromise for software based conversion in typical use cases as with xvimagesink.

For the sake of the tests, I disabled those extra color and some tests got fixed, but some other tests (with big resolutions) where terribly slow, generating timeouts in fluster. I realized that it was because of derived images. Right now va plugin is quite aggressive using derived images. @haihao warned us about this in the past. So I disabled derived images, and the software-based conversion turned as fast as expected.

Results with default surface format (NV12) and no derived images:

	VA	VAAPI
H264	129/135	126/135
H265	142/147	142/147
VP9	190/303	216/303
VP8	57/61	57/61

I would like to have the numbers for i915 driver and amd gallium, but for sake of correctness, using the default color format and disabling derived for frame reading, looks like a "good think".