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

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

drivers/gpu/drm/gma500/oaktrail_hdmi.c: In function htotal_calculate:
drivers/gpu/drm/gma500/oaktrail_hdmi.c:160:6: warning: variable htotal set but not used [-Wunused-but-set-variable]

It is introduced by commit 39ec748f ("gma600: Enable HDMI support"),
but 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/1573828027-122323-2-git-send-email-zhengbin13@huawei.com