drm/i915: Tidy workaround batch buffer emission

Use the "*batch++ = " style as in the ring emission for better
readability and also simplify the logic a bit by consolidating
the offset and size calculations and overflow checking. The
latter is a programming error so it is not required to check
for it after each write to the object, but rather do it once the
whole state has been written and fail the driver if something
went wrong.

v2: Rebase.

v3: Keep track of offsets and sizes in bytes for simplicity
    and rename function pointer variable to _fn suffix.
    (Chris Wilson)

v4: Fix size calc broken in v3 and add alignment warning. (Chris Wilson)

v5: Fix return code.

v6: I added an exit from loop in v5 but forgot to put back
    the object teardown.
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk> (v5)
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>