st/dri: fix OpenGL-OpenCL interop for GL_TEXTURE_BUFFER

Tested by our OpenCL team.

Fixes: 9c499e67 "st/mesa: don't invoke st_finalize_texture & st_convert_sampler for TBOs"

Acked-by: Alex Deucher <alexander.deucher@amd.com>
(cherry picked from commit db495b89)
[Juan A. Suarez: resolve trivial conflicts]
Signed-off-by: Juan A. Suarez Romero <jasuarez@igalia.com>

Conflicts:
	src/gallium/state_trackers/dri/dri2.c