i915: Don't do the pitch expansion for tiled buffers.

i915: Don't do the pitch expansion for tiled buffers.

The weirdness that led to the bumping of pitch for those 512/1024
pixels is that in taking a 2x2 subspan or bilinear filtering we'd end
up hitting the same channel in 2 different pages, leading to lower
performance.  With tiling, that doesn't occur, so we don't need to
waste the memory.