Linking fails when not writing gl_Position.

Linking fails when not writing gl_Position.

According to GLSL-ES Spec(i.e. 1.0, 3.0), gl_Position value is undefined
after the vertex processing stage if we don't write gl_Position. However,
GLSL 1.10 Spec mentions that writing to gl_Position is mandatory. In case
of GLSL-ES, it's not an error and atleast the linking should pass.
Currently, Mesa throws an linker error in case we dont write to gl_position
and Version is less then 140(GLSL) and 300(GLSL-ES). This patch changes
it so that we don't report an error in case of GLSL-ES.

Signed-off-by: Kalyan Kondapally <kalyan.kondapally@intel.com>
Reviewed-by: Tapani Pälli <tapani.palli@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=83380