gallium/dri: Support DRI Image extension version 6
gallium/dri: Support DRI Image extension version 6

v2: Pick out the correct gl_context pointer
v3: Don't leak pipe_resources on error path
    Set img->dri_format correctly

Signed-off-by: Christopher James Halse Rogers <f4edbc39080c6cd998e1a8e915acbce7f6e0704a@canonical.com>
Reviewed-by: Thomas Hellstrom <a26490dfbee4be3d38287ad6448444504f58193c@vmware.com>
Signed-off-by: Maarten Lankhorst <743c2818a341d71b99ed2b566f9f80c2fc288f93@canonical.com>

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