drm/gma500: remove set but not used variable 'channel_eq'

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

drivers/gpu/drm/gma500/cdv_intel_dp.c: In function cdv_intel_dp_complete_link_train:
drivers/gpu/drm/gma500/cdv_intel_dp.c:1596:7: warning: variable channel_eq set but not used [-Wunused-but-set-variable]

It is never used, so remove it.

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: zhengbin <zhengbin13@huawei.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/1573902268-117518-1-git-send-email-zhengbin13@huawei.com