llvmpipe: enable clear_texture with util_clear_texture

Passes all corresponding piglit tests.

Signed-off-by: Lars Hamre <chemecse@gmail.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Reviewed-by: Edward O'Callaghan <funfunctor@folklore1984.net>

src/gallium/drivers/llvmpipe/lp_screen.c

       return 1;
    case PIPE_CAP_COPY_BETWEEN_COMPRESSED_AND_PLAIN_FORMATS:
       return 1;
+   case PIPE_CAP_CLEAR_TEXTURE:
+      return 1;
    case PIPE_CAP_MULTISAMPLE_Z_RESOLVE:
    case PIPE_CAP_RESOURCE_FROM_USER_MEMORY:
    case PIPE_CAP_DEVICE_RESET_STATUS_QUERY:
    case PIPE_CAP_TGSI_TXQS:
    case PIPE_CAP_FORCE_PERSAMPLE_INTERP:
    case PIPE_CAP_SHAREABLE_SHADERS:
-   case PIPE_CAP_CLEAR_TEXTURE:
    case PIPE_CAP_DRAW_PARAMETERS:
    case PIPE_CAP_TGSI_PACK_HALF_FLOAT:
    case PIPE_CAP_MULTI_DRAW_INDIRECT:

src/gallium/drivers/llvmpipe/lp_surface.c

    lp->pipe.clear_depth_stencil = llvmpipe_clear_depth_stencil;
    lp->pipe.create_surface = llvmpipe_create_surface;
    lp->pipe.surface_destroy = llvmpipe_surface_destroy;
-   /* These two are not actually functions dealing with surfaces */
+   /* These are not actually functions dealing with surfaces */
+   lp->pipe.clear_texture = util_clear_texture;
    lp->pipe.resource_copy_region = lp_resource_copy;
    lp->pipe.blit = lp_blit;
    lp->pipe.flush_resource = lp_flush_resource;