i965: Improve same-buffer restriction for imports

i965: Improve same-buffer restriction for imports

Intel hardware requires that all planes of an image come from the same
buffer, which is currently implemented by testing that all FDs are
numerically the same.

However, when going through a winsys (e.g.) or anything which transits
FDs individually, the FDs may be different even if the underlying buffer
is the same.

Instead of checking the FDs for equality, we must check if they actually
point to the same buffer (Jason).

Reviewed-by: Varad Gautam <varad.gautam@collabora.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>