i965: Drop the aux mt when not used

i965: Drop the aux mt when not used

This patch will preserve the BO & offset, and not the miptree for the
aux_mcs buffer. Eventually it might make sense to pull put the sizing
function in miptree creation, but for now this should be sufficient
and not too hideous.

v2: Save BO's offset too (Lionel)

v3: Squash previous patch storing the size of the allocated aux buffer
    (Lionel)
    Fix memory leak with mcs_buf->bo (Lionel)

Signed-off-by: Ben Widawsky <benjamin.widawsky@intel.com> (v1)
Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com> (v2)
Reviewed-by: Topi Pohjolainen <topi.pohjolainen@intel.com>