i965/blit: Remove a bogus assertion

i965/blit: Remove a bogus assertion

This assertion, while valid for linear buffers, doesn't work properly for
tiled memory.  It used to work most of the time because the offset provided
was always to the left-hand edge of the image.  However, if you use a byte
offset to get to the inside of the image, the height * stride calculation
may actually end up being too large.

Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Topi Pohjolainen <topi.pohjolainen@intel.com>
Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>