llvmpipe: improve alignment calculation for fetching/storing pixels

llvmpipe: improve alignment calculation for fetching/storing pixels

This was always doing per-pixel alignment which isn't necessary, except
for the buffer case (due to the per-element offset). The disabled code
for calculating it was incorrect because it assumed that always the full
block would be fetched, which may not be the case, so fix this up.
The original code failed for instance for r10g10b10a2 the alignment would
have been calculated as 4 (block_width) * 4 (bytes) so 16, but the actual
fetch may have only fetched 2 values at a time, hence only alignment 8 -
it is unclear what exactly would happen in this case (alignment larger
than size to fetch).
So just use the (already calculated) fetch size instead and get alignment
from that which should always work, no matter if fetching 1,2 or 4 pixels.

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>