mesa: Refactor error checking for GL_TEXTURE_BASE_LEVEL vs texture targets

Add a big spec quotation justifying the error generated, which has
changed over the GL versions.

v2: Compact the spec quote based on a Khronos bug and discussion with Jason.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>
1 file changed