gles1: fix glBufferSubData()
This enables InternalBufferSubDataCopyMESA function for OpenGL ES 1.1 contexts. This fixes glBufferSubData() when PIPE_CAP_ALLOW_GLTHREAD_BUFFER_SUBDATA_OPT is enabled such as in radeonsi.
_mesa_marshal_BufferSubData_merged may use InternalBufferSubDataCopyMESA if using AllowGLThreadBufferSubDataOpt (PIPE_CAP_ALLOW_GLTHREAD_BUFFER_SUBDATA_OPT). There is no check if the InternalBufferSubDataCopyMESA function is allowed; it is assumed all OpenGL contexts support it.
Fixes #11368 (closed).