i965/tiled_memcpy: linear_to_ytiled a cache line at a time

i965/tiled_memcpy: linear_to_ytiled a cache line at a time

TileY's low 6 address bits are: v1 v0 u3 u2 u1 u0
Thus a cache line in the tiled surface is composed of a 2d area of
16x4 bytes of the linear surface.

Add a special case where the area being copied is 4-line aligned
and a multiple of 4-lines so that entire cache lines will be
written at a time.

On Apollolake, this increases tiling throughput to wc maps by
84.0103% +/- 0.862818%

v2: Split [y0, y1) and [y2, y3) loops apart for clarity (Jason Ekstrand)
v3: Don't reset src var (Jason), Ensure y0 <= y1 <= y2 <= y3

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>