v3d: Store the actual mask of color buffers present in the key.

If you only bound rt 1+, we'd still emit a write to the rt0 that isn't
present (noticed while debugging an
ext_framebuffer_multisample-alpha-to-coverage-no-draw-buffer-zero
regression in another change).

src/broadcom/compiler/v3d_compiler.h

@@ -357,7 +357,8 @@ struct v3d_fs_key {
         bool sample_alpha_to_one;
         bool clamp_color;
         bool shade_model_flat;
-        uint8_t nr_cbufs;
+        /* Mask of which color render targets are present. */
+        uint8_t cbufs;
         uint8_t swap_color_rb;
         /* Mask of which render targets need to be written as 32-bit floats */
         uint8_t f32_color_rb;