i965: Add more stringent blitter assertions

i965: Add more stringent blitter assertions

Blits to or from a y-tiled surface must always be a multiple of the tile size.
From page 16 of the HSW PRM
(https://01.org/linuxgraphics/sites/default/files/documentation/intel-gfx-prm-osrc-hsw-memory-views.pdf#16)
"The pitch of a tiled enclosing region must be an integral number of tile
widths"

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>