disp: msm: sde: unset and set clk parents during pm_suspend

Currently link clk parent set/unset are done as part of dsi_prepare
and dsi_unprepare, but in case of deepsleep with display ON, these
will not be called. Due to mismatch in parent between clk
framework and actual parent, subsequent clk_set_parent will
early return without setting the parent.

To avoid this condition, do set parent to xo, as part of
pm_suspend and restore to link clk source in pm_resume.

Change-Id: I626899304580f9d9fbcc92cd8b139cd89cd48999
Signed-off-by: Venkata Prahlad Valluru <quic_vvalluru@quicinc.com>