v3d: handle writes to gl_Layer from geometry shaders

When geometry shaders write a value to gl_Layer that doesn't correspond to
an existing layer in the target framebuffer the rendering behavior is
undefined according to the spec, however, there are CTS tests that trigger
this scenario on purpose, probably to ensure that nothing terrible happens.

For V3D, this situation is problematic because the binner uses the layer
index to select the offset to write into the tile state data, and we only
allocate tile state for MAX2(num_layers, 1), so we want to make sure we
don't produce values that would lead to out of bounds writes. The simulator
has an assert to catch this, although we haven't observed issues in actual
hardware it is probably best to play safe.

Reviewed-by: Alejandro Piñeiro <apinheiro@igalia.com>

src/broadcom/compiler/v3d_compiler.h

@@ -279,6 +279,14 @@ enum quniform_contents {
          * L2T cache will effectively be the shared memory area.
          */
         QUNIFORM_SHARED_OFFSET,
+
+        /**
+         * Returns the number of layers in the framebuffer.
+         *
+         * This is used to cap gl_Layer in geometry shaders to avoid
+         * out-of-bounds accesses into the tile state during binning.
+         */
+        QUNIFORM_FB_LAYERS,
 };
 
 static inline uint32_t v3d_unit_data_create(uint32_t unit, uint32_t value)