drm/nouveau/kms/nv04: remove set but not used variable 'width'

drivers/gpu/drm/nouveau/dispnv04/arb.c: In function nv04_calc_arb:
drivers/gpu/drm/nouveau/dispnv04/arb.c:56:21: warning:
 variable width set but not used [-Wunused-but-set-variable]

'width' is never used, so remove it.

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>