The "src_height" computed in save_deblock_boundary_lines didn't match
the one in save_tile_row_boundary_lines, which meant that the wrapper
function assumed the deblock code was saving some lines and that code
thought that save_cdef_boundary_lines would do it.

This patch fixes up the logic to match, and also completely gets rid
of the lines_to_save variable (after all, bad things would happen if
lines_to_save was 1 because we'll still read both boundary lines
later)

The tile height gets rounded up to a multiple of 8 luma pixels in
save_tile_row_boundary_lines to avoid nasty corner cases. This will
only have any effect for rows at the bottom of the frame (where
av1_get_tile_rect clips to the frame boundary).

BUG=aomedia:1020

Change-Id: I55adb53fa8ba9c7f97fb2fd5b328a3f2f5065464