drm/radeon/r600: fix offset handling in CS parser

Need add reloc offset to the offset in the actual
packet.  Fixes use of the DRAW_INDEX packet by the 3D
driver.

[airlied: modified first one where idx_value == ib[idx+0]

Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@linux.ie>