avcenc: Fix the gcc inline assembly constraints

Use +r for registers whose value is used as input to the block.

Also switch input registers to output registers with +r, for
registers that are modified by the assembly block (SUB_SAD in
sad_inline.h).

Without this, the inline assembly fails on modern gcc versions
(tested on gcc 4.6), where the intended input value never is
loaded into the registers, causing segfaults at runtime.

Currently this assembly isn't used, but this change makes it
work properly if enabled.

Change-Id: I37c247cd5c8aff213445e31ab3bd33d8f0f37e05