drm/amd/display: Remove redundant initialization of dpg_width

dpg_width is being initialized to width but this is never read
as dpg_width is overwritten later on. Remove the redundant
initialization.

Cleans up the following clang-analyzer warning:

drivers/gpu/drm/amd/amdgpu/../display/dc/core/dc_link_dp.c:6020:8:
warning: Value stored to 'dpg_width' during its initialization is never
read [clang-analyzer-deadcode.DeadStores].

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>