drm: samsung: set power.must_resume to true in the suspend path

dsim_suspend/resume relies on matching pair of calls to prevent
stuck in a specific state. Need to set the power.must_resume
status bit to true to avoid skipping resume.

Bug: 196158811
Test: idle in AOD mode without USB connection
Change-Id: I6e52621309d9777182e9a9f871dae05974f75a17
Signed-off-by: Chris Lu <luchris@google.com>