freedreno/a6xx: Add register offset for STG/LDG

freedreno/a6xx: Add register offset for STG/LDG

These instructions take a 64 bit iova as two conescutive registers and
a immediate offset.  This patch adds support for the offset to be a
single register, which is added to the 64 bit iova.

Signed-off-by: Kristian H. Kristensen <hoegsberg@google.com>
Acked-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Rob Clark <robdclark@gmail.com>