i965: Fix aligning to the block size in intel_miptree_copy_slice

i965: Fix aligning to the block size in intel_miptree_copy_slice

This function was trying to align the width and height to a multiple
of the block size for compressed textures. It was using align_w/h as a
shortcut to get the block size as up until Gen9 this always happens to
match. However in Gen9+ the alignment values are expressed as
multiples of the block size so in effect the alignment values are
always 4 for compressed textures as that is the minimum value we can
pick. This happened to work for most compressed formats because the
block size is also 4, but for FXT1 this was breaking because it has a
block width of 8.

This fixes some Piglit tests testing FXT1 such as

spec@3dfx_texture_compression_fxt1@fbo-generatemipmap-formats

Reviewed-by: Nanley Chery <nanley.g.chery@intel.com>