drm/i915/blt: fixup block_size rounding

There is nothing to say that the obj->base.size is actually a multiple
of the block_size.

v2: Use round_up() as block_size is a power-of-two

Reported-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20191028220325.9325-1-chris@chris-wilson.co.uk