drm/amdgpu/display: initialize the variable 'i'

>> drivers/gpu/drm/amd/amdgpu/../display/amdgpu_dm/amdgpu_dm.c:9804:38:
>> warning: variable 'i' is uninitialized when used here
>> [-Wuninitialized]
                           timing  = &edid->detailed_timings[i];
                                                             ^
   drivers/gpu/drm/amd/amdgpu/../display/amdgpu_dm/amdgpu_dm.c:9714:7:
note: initialize the variable 'i' to silence this warning
           int i;
                ^
                 = 0
   1 warning generated.

Initialize the variable 'i'.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Souptick Joarder <jrdr.linux@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>