mesa/main: fix gl_buffer_index enum comparison

mesa/main: fix gl_buffer_index enum comparison

For clang, enums are unsigned by default and gives the following warning:

external/mesa3d/src/mesa/main/buffers.c:764:21: warning: comparison of constant -1 with expression of type 'gl_buffer_index' is always false [-Wtautological-constant-out-of-range-compare]
      if (srcBuffer == -1) {
          ~~~~~~~~~ ^  ~~

Replace -1 with an enum value to fix this.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Signed-off-by: Rob Herring <robh@kernel.org>