drm/amd/display: remove set but not used variable 'msg_out'

drivers/gpu/drm/amd/amdgpu/../display/modules/hdcp/hdcp_psp.c: In function mod_hdcp_hdcp2_enable_encryption:
drivers/gpu/drm/amd/amdgpu/../display/modules/hdcp/hdcp_psp.c:633:77: warning: variable msg_out set but not used [-Wunused-but-set-variable]
drivers/gpu/drm/amd/amdgpu/../display/modules/hdcp/hdcp_psp.c: In function mod_hdcp_hdcp2_enable_dp_stream_encryption:
drivers/gpu/drm/amd/amdgpu/../display/modules/hdcp/hdcp_psp.c:710:77: warning: variable msg_out set but not used [-Wunused-but-set-variable]

It is never used, so remove it.

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>