i965/fs: Return mlen * 8 for size_read() for INTERPOLATE_AT_*
They are send messages and this makes size_read() and mlen agree. For
both of these opcodes, the payload is just a dummy so mlen == 1 and this
should decrease register pressure a bit.
Reviewed-by:
Francisco Jerez <currojerez@riseup.net>
Cc: mesa-stable@lists.freedesktop.org