drm/amd/display: remove set but not used variable 'pixel_width'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/gpu/drm/amd/display/dc/dcn10/dcn10_dpp.c:137:11: warning: variable ‘pixel_width’ set but not used [-Wunused-but-set-variable]

It is introduced by commit 70ccab60 ("drm/amdgpu/display:
Add core dc support for DCN"), but never used, so remove it.

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Zheng Bin <zhengbin13@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>