v3d: Explicitly expose OpenGL ES Shading Language 3.1

This will expose GL_EXT_primitive_bounding_box and
GL_OES_primitive_bounding_box after previous commits
expose OpenGL ES 3.1 once Compute Shaders are available.

Reviewed-by: Eric Anholt <eric@anholt.net>

src/gallium/drivers/v3d/v3d_screen.c

@@ -183,6 +183,9 @@ v3d_screen_get_param(struct pipe_screen *pscreen, enum pipe_cap param)
         case PIPE_CAP_GLSL_FEATURE_LEVEL:
                 return 330;
 
+        case PIPE_CAP_ESSL_FEATURE_LEVEL:
+                return 310;
+
 	case PIPE_CAP_GLSL_FEATURE_LEVEL_COMPATIBILITY:
 		return 140;