drm/amd/amdgpu/ih_v6_0: Repair misspelling and provide descriptions for 'ih'

drm/amd/amdgpu/ih_v6_0: Repair misspelling and provide descriptions for 'ih'

Fixes the following W=1 kernel build warning(s):

 drivers/gpu/drm/amd/amdgpu/ih_v6_0.c:392: warning: Function parameter or member 'ih' not described in 'ih_v6_0_get_wptr'
 drivers/gpu/drm/amd/amdgpu/ih_v6_0.c:432: warning: Function parameter or member 'ih' not described in 'ih_v6_0_irq_rearm'
 drivers/gpu/drm/amd/amdgpu/ih_v6_0.c:458: warning: Function parameter or member 'ih' not described in 'ih_v6_0_set_rptr'

Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: "Christian König" <christian.koenig@amd.com>
Cc: "Pan, Xinhui" <Xinhui.Pan@amd.com>
Cc: David Airlie <airlied@gmail.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Hawking Zhang <Hawking.Zhang@amd.com>
Cc: amd-gfx@lists.freedesktop.org
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Lee Jones <lee@kernel.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>