Reduce the Vulkan interop buffer kernel workgroup size to 256

Using `local_size_x = 512` in the shader used in the Vulkan interop
tests fails on older hardware because this exceeds the maximum barrier
size limit.

Changed to 256, which is still a multiple of 32 and 64,
and is below the hardware limitations. The tests are not performance
tests anyways, so the performance penalty, if any, is acceptable.

Signed-off-by: Ahmed Hesham <ahmed.hesham@arm.com>

test_conformance/vulkan/shaders/buffer.comp

@@ -15,7 +15,7 @@ layout(binding = 1) buffer Buffer
 {
   uint8_t ptr[];
 } bufferPtrList[MAX_BUFFERS];
-layout(local_size_x = 512) in;
+layout(local_size_x = 256) in;
 void main() {
     for (uint32_t bufIdx = 0; bufIdx < numBuffers; bufIdx++) {
         uint32_t ptrIdx = gl_GlobalInvocationID.x;