llvmpipe: enable PIPE_CAP_INDEP_BLEND_FUNC

The driver was saying that independend blend functions was not supported,
but it really was.  The driver was using the per-target independend blend
factors but the state tracker was only setting the 0th one (per the
Gallium spec).

Fixes a piglit fbo-drawbuffers2-blend regression.
See https://bugs.freedesktop.org/show_bug.cgi?id=33215

src/gallium/drivers/llvmpipe/lp_screen.c

@@ -151,7 +151,7 @@ llvmpipe_get_param(struct pipe_screen *screen, enum pipe_cap param)
    case PIPE_CAP_INDEP_BLEND_ENABLE:
       return 1;
    case PIPE_CAP_INDEP_BLEND_FUNC:
-      return 0;
+      return 1;
    case PIPE_CAP_TGSI_FS_COORD_ORIGIN_UPPER_LEFT:
    case PIPE_CAP_TGSI_FS_COORD_PIXEL_CENTER_INTEGER:
       return 1;