drm/amdgpu/display: make a const array common_rates static, makes object smaller

Don't populate the const array common_rates on the stack but instead it
static. Makes the object code smaller by 80 bytes:

Before:
   text	   data	    bss	    dec	    hex	filename
 268019	  98322	    256	 366597	  59805	../display/amdgpu_dm/amdgpu_dm.o

After:
   text	   data	    bss	    dec	    hex	filename
 267843	  98418	    256	 366517	  597b5	../display/amdgpu_dm/amdgpu_dm.o

Reduction of 80 bytes

(gcc version 10.3.0)

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>