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

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

drivers/gpu/drm/amd/display/dc/dcn20/dcn20_dpp.c: In function dpp2_get_optimal_number_of_taps:
drivers/gpu/drm/amd/display/dc/dcn20/dcn20_dpp.c:359:11: warning: variable pixel_width set but not used [-Wunused-but-set-variable]

It is not used since commit f7de96ee ("drm/amd/display:
Add DCN2 DPP")

Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: zhengbin <zhengbin13@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>