drm/i915/dsi: remove set but not used variable 'hfront_porch'

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

drivers/gpu/drm/i915/display/icl_dsi.c: In function 'gen11_dsi_set_transcoder_timings':
drivers/gpu/drm/i915/display/icl_dsi.c:768:6: warning:
 variable 'hfront_porch' set but not used [-Wunused-but-set-variable]

It is never used and can be removed.

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190719015136.103988-1-yuehaibing@huawei.com