gallium: add flatshade lowering capability

This allows the driver to request flatshade lowering.
(NIR drivers only so far).

Reviewed-by: Marek Olšák <marek.olsak@amd.com>

src/gallium/auxiliary/util/u_screen.c

@@ -394,6 +394,9 @@ u_pipe_screen_get_param_defaults(struct pipe_screen *pscreen,
    case PIPE_CAP_TEXTURE_SHADOW_MAP: /* Enables ARB_shadow */
       return 1;
 
+   case PIPE_CAP_FLATSHADE:
+      return 1;
+
    default:
       unreachable("bad PIPE_CAP_*");
    }

src/gallium/docs/source/screen.rst

@@ -558,6 +558,7 @@ The integer capabilities:
 * ``PIPE_CAP_GL_SPIRV_VARIABLE_POINTERS``: True if the driver supports Variable Pointers in SPIR-V shaders.
 * ``PIPE_CAP_DEMOTE_TO_HELPER_INVOCATION``: True if driver supports demote keyword in GLSL programs.
 * ``PIPE_CAP_TGSI_TG4_COMPONENT_IN_SWIZZLE``: True if driver wants the TG4 component encoded in sampler swizzle rather than as a separate source.
+* ``PIPE_CAP_FLATSHADE``: Driver supports pipe_rasterizer_state::flatshade.
 
 .. _pipe_capf:
 

src/gallium/include/pipe/p_defines.h

@@ -904,6 +904,7 @@ enum pipe_cap
    PIPE_CAP_GL_SPIRV_VARIABLE_POINTERS,
    PIPE_CAP_DEMOTE_TO_HELPER_INVOCATION,
    PIPE_CAP_TGSI_TG4_COMPONENT_IN_SWIZZLE,
+   PIPE_CAP_FLATSHADE,
 };
 
 /**