radv: fix dual source blending

radv: fix dual source blending

Add the index to the location when assigning driver locations for
output variables.

Otherwise two fragment shader outputs declared as:

   layout (location = 0, index = 0) out vec4 output1;
   layout (location = 0, index = 1) out vec4 output2;

will end up aliasing one another.

Note that this patch will make the second output variable in the above
example alias a possible third output variable with location = 1 and
index = 0. But this shouldn't be a problem in practice since only one
color attachment is supported when dual-source blending is used.

Cc: "13.0" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>