mesa: fix misaligned pointer returned by dlist_alloc

mesa: fix misaligned pointer returned by dlist_alloc

In cases where the to-be-allocated node size with padding exceeds BLOCK_SIZE
but without padding doesn't, a new block is not created and no padding is done
to the previous instruction, causing a misaligned pointer to be returned.

v2: Per Ilia Mirkin's suggestion, remove the extra condition in the first
    if statement, let it unconditionally pad the last instruction if needed.
    The updated currentPos will then be taken into account in the
    block size checking.

This fixes crash seen with lightsmark and Optuma apitraces

Fixes:  05605d7f537c4 (' mesa: remove display list OPCODE_NOP')

Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Neha Bhende <bhenden@vmware.com>
Tested-by: Neha Bhende <bhenden@vmware.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/14871>