drm/amdgpu: suppress the warning about enum value 'AMD_IP_BLOCK_TYPE_NUM'

Suppress the warning below on building htmldocs:
drivers/gpu/drm/amd/include/amd_shared.h:103: warning: Enum value
'AMD_IP_BLOCK_TYPE_NUM' not described in enum 'amd_ip_block_type'

Fixes: 6ee27ee2 ("drm/amd/pm: avoid duplicate powergate/ungate setting")

Signed-off-by: Evan Quan <evan.quan@amd.com>
Reviewed-by: Guchun Chen <guchun.chen@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>