i965: Apply depthstencil alignment workaround when doing fast clears.

Fast depth clears have the same depth/stencil alignment requirements
as other drawing operations.  Therefore, we need to call
brw_workaround_depthstencil_alignment() from both the clear and
drawing paths.

Without this fix, we get image corruption if the following conditions
hold: (a) the first ever drawing operation to a depth miplevel (or the
first drawing operation after having used the texture for sampling) is
a clear, (b) the depth miplevel has a size that is eligible for fast
depth clears, and (c) the depth miplevel has an offset within the
miptree that isn't 8x8 aligned.

Fixes piglit "depthstencil-render-miplevels" tests with size 273.

NOTE: This is a candidate for stable branches