drm/i915/dp: use true,false for bool variable in intel_dp.c

drm/i915/dp: use true,false for bool variable in intel_dp.c

Fixes coccicheck warning:

drivers/gpu/drm/i915/display/intel_dp.c:4950:1-33: WARNING: Assignment of 0/1 to bool variable
drivers/gpu/drm/i915/display/intel_dp.c:4906:1-33: WARNING: Assignment of 0/1 to bool variable

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Ma Feng <mafeng.ma@huawei.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1578013959-31486-3-git-send-email-mafeng.ma@huawei.com