drirc: Add glsl_zero_init workaround for GpuTest

drirc: Add glsl_zero_init workaround for GpuTest

GiMark benchmark from GpuTest has such code in VS:

 out vec4 lightDir0;
 out vec4 lightDir1;

 ...

 lightDir0.xyz = lp0 - vVertex.xyz;
 lightDir1.xyz = lp1 - vVertex.xyz;

In FS:

 float distSqr = dot(lightDir0, lightDir0);

So due to the usage of uninitialized .w channel in the dot product,
distSqr may become undefined which results in many black dots
in the test on Iris.

In https://www.geeks3d.com/forums/index.php/topic,6242.0.html
developer stated that this benchmark most likely won't be updated.

Closes: https://gitlab.freedesktop.org/mesa/mesa/issues/1919
Signed-off-by: Danylo Piliaiev <danylo.piliaiev@globallogic.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>