drm/amd/display/amdgpu_dm: remove duplicate included header files

soc15_common.h is included more than once.

Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: Xu Panda <xu.panda@zte.com.cn>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>