Remove duplicate exa_wm_yuv_rgb.g5a shader source

exa_wm_yuv_rgb.g5a is identical to exa_wm_yuv_rgb.g4a. Just use a
symlink intead of duplicating the whole file.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>