drm/msm/dp: change mdss_dp_active and mdss_dp_sleep to optional

As the following new pinctrl states have been added for DP:

mdss_dp_hpd_tlmm
mdss_dp_hpd_ctrl

We need to make the following states also optional, in case if
only the above new states are defined:

mdss_dp_active
mdss_dp_sleep

CRs-Fixed: 2414578
Change-Id: I84098fb5a4aa7f73726c01593f93b173aaee0758
Signed-off-by: Xiaowen Wu <wxiaowen@codeaurora.org>