v4l2object: if colorimetry matrix in caps is RGB, use the default one

When colorimetry matrix is GST_VIDEO_COLOR_MATRIX_RGB in caps from demux, this type is cast to V4L2_YCBCR_ENC_DEFAULT(0) type in set format. And V4L2_YCBCR_ENC_DEFAULT type is cast to GST_VIDEO_COLOR_MATRIX_BT709 or GST_VIDEO_COLOR_MATRIX_BT601 and so on in gst_v4l2_object_get_colorspace.This will lead to inconsistencies in the GstVideoColorimetry before and after, resulting in a match failure.