drm/amdkfd: Fix reg offset for setting CWSR grace period

This patch fixes the case where the code currently passes
absolute register address and not the reg offset, which HWS
expects, when sending the PM4 packet to set/update CWSR grace
period. Additionally, cleanup the signature of
build_grace_period_packet_info function as it no longer needs
the inst parameter.

Signed-off-by: Mukul Joshi <mukul.joshi@amd.com>
Reviewed-by: Jonathan Kim <jonathan.kim@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>