i965/wm: surfaces should have the API buffer size, not the drm buffer size

i965/wm: surfaces should have the API buffer size, not the drm buffer size

The returned drm buffer object has a size multiple of 4096 but that should not
be exposed to the API user, which is working with a different size.

As far as I can see this problem is only visible in the calculation of the
length of unsized arrays used in SSBOs, as the implementation of this needs
to query the underlying buffer size via a message.

Signed-off-by: Samuel Iglesias Gonsalvez <siglesias@igalia.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Kristian Høgsberg <krh@bitplanet.net>