glupload: refine transform caps of directviv uploade

Submitted by Haihua Hu @JaredHu

Link to original bug (#797002)

Description

if downstream only accept YUV format (eg, add capsfilter), then we should passthrough color format and don't extend to directviv support format.

The failed case is:

gst-launch-1.0 videotestsrc ! glupload ! video/x-raw(memory:GLMemory),format=I420 ! glcolorconvert ! video/x-raw(memory:GLMemory),format=RGBA ! glimagesinkelement