drm/i915: Remove set but not used variable 'encoder'

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

drivers/gpu/drm/i915/display/intel_dp.c: In function 'intel_dp_set_drrs_state':
drivers/gpu/drm/i915/display/intel_dp.c:6623:24: warning:
 variable 'encoder' set but not used [-Wunused-but-set-variable]

It's never used, so can be removed.Also remove related
variable 'dig_port'

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190705113112.64715-1-yuehaibing@huawei.com