drm/amd/amdgpu/tonga_ih: Provide some missing descriptions for 'ih' and 'entry'

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

 drivers/gpu/drm/amd/amdgpu/tonga_ih.c:193: warning: Function parameter or member 'ih' not described in 'tonga_ih_get_wptr'
 drivers/gpu/drm/amd/amdgpu/tonga_ih.c:225: warning: Function parameter or member 'ih' not described in 'tonga_ih_decode_iv'
 drivers/gpu/drm/amd/amdgpu/tonga_ih.c:225: warning: Function parameter or member 'entry' not described in 'tonga_ih_decode_iv'
 drivers/gpu/drm/amd/amdgpu/tonga_ih.c:255: warning: Function parameter or member 'ih' not described in 'tonga_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: 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>