drm/amd/amdgpu/vega20_ih: Add missing descriptions for 'ih' and fix spelling error

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

 drivers/gpu/drm/amd/amdgpu/vega20_ih.c:378: warning: Function parameter or member 'ih' not described in 'vega20_ih_get_wptr'
 drivers/gpu/drm/amd/amdgpu/vega20_ih.c:421: warning: Function parameter or member 'ih' not described in 'vega20_ih_irq_rearm'
 drivers/gpu/drm/amd/amdgpu/vega20_ih.c:447: warning: Function parameter or member 'ih' not described in 'vega20_ih_set_rptr'

Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: "Christian König" <christian.koenig@amd.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Hawking Zhang <Hawking.Zhang@amd.com>
Cc: Feifei Xu <Feifei.Xu@amd.com>
Cc: amd-gfx@lists.freedesktop.org
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>