i965: bump MAX_DEPTH_TEXTURE_SAMPLES to 4/8

Bump MAX_DEPTH_TEXTURE_SAMPLES to match what GetInternalformativ is
claiming. Since that limit is what is actually enforced now, this
doesn't actually change anything except the queried value.

There's still no piglits verifying that multisample depth textures work,
but this works in the Unigine demos.

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>