u_upload_mgr: rework the application of alignment

u_upload_mgr: rework the application of alignment

The function only aligned the size, but not the offset.
The offset was aligned only when the previous suballocation was aligned.
That yielded the correct offset alignment if the alignment was constant
for all suballocations.

Instead, directly align the offset, but allow an unaligned size.
There is no change in behavior, because the alignment is constant
at the moment.

This a prerequisite for allowing a variable alignment for suballocations.

Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>