gallium: Add a cap to check if the driver supports ARB_post_depth_coverage

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>
diff --git a/src/gallium/drivers/i915/i915_screen.c b/src/gallium/drivers/i915/i915_screen.c
index 1cf9441..a0dbc12 100644
--- a/src/gallium/drivers/i915/i915_screen.c
+++ b/src/gallium/drivers/i915/i915_screen.c
@@ -277,6 +277,7 @@
    case PIPE_CAP_POLYGON_OFFSET_UNITS_UNSCALED:
    case PIPE_CAP_TGSI_ARRAY_COMPONENTS:
    case PIPE_CAP_POLYGON_MODE_FILL_RECTANGLE:
+   case PIPE_CAP_POST_DEPTH_COVERAGE:
       return 0;
 
    case PIPE_CAP_MAX_DUAL_SOURCE_RENDER_TARGETS: