drm/amd/amdgpu: Clean up gfx_v8_0_kiq_set_interrupt_state()

Use new WREG32_FIELD_OFFSET() to clean up code.

Signed-off-by: Tom St Denis <tom.stdenis@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>