Enable amdgpu support for SI parts

configname: CONFIG_DRM_AMDGPU_SI

Linux Kernel Configuration
└─>Device Drivers
└─>Graphics support
└─>Enable amdgpu support for SI parts
In linux kernel since version 3.10 (release Date: 2013-06-30)  
Choose this option if you want to enable experimental support
for SI (Southern Islands) asics.

SI is already supported in radeon. Experimental support for SI
in amdgpu will be disabled by default and is still provided by
radeon. Use module options to override this:

radeon.si_support=0 amdgpu.si_support=1