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 (cherry picked from commit de1f22d5)