llvmpipe: Use the default behavior of ALLOW_MAPPED_BUFFERS.

Since this is a software rasterizer, we really don't care whether the
buffers are "mapped" since it's just malloc.  This will drop a bit of
pointless CPU overhead to throw errors.

Reviewed-by: Dave Airlie <airlied@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/3298>

src/gallium/drivers/llvmpipe/lp_screen.c

@@ -319,7 +319,6 @@ llvmpipe_get_param(struct pipe_screen *screen, enum pipe_cap param)
    case PIPE_CAP_PCI_DEVICE:
    case PIPE_CAP_PCI_FUNCTION:
    case PIPE_CAP_GLSL_OPTIMIZE_CONSERVATIVELY:
-   case PIPE_CAP_ALLOW_MAPPED_BUFFERS_DURING_EXECUTION:
       return 0;
    case PIPE_CAP_MAX_GS_INVOCATIONS:
       return 32;