cogl-gst: video-sink: fix YV12/I420 support

This fixes a mistake in commit 637728dd89d51bc28 that was meant to fix
yv12/i420 support but when the patch was updated based on review to use
COGL_PIXEL_FORMAT_A instead of COGL_PIXEL_FORMAT_G the corresponding
glsl code wasn't also updated to sample the .a component.