drm_hwcomposer: Fixes for display hotplug / headless mode

Further testing showed that several issues is still present:
1. Boot without display doesn't work.
2. Unplug/plug primary display has some flaws due to incomplete
   HwcDisplay disposal.
3. In case creation of the pipeline fails, hwcomposer crashes.

This commit aims to address them.

Fixes: bb594baa ("drm_hwcomposer: Rework HwcDisplay disposal to avoid races")
Signed-off-by: Roman Stratiienko <roman.o.stratiienko@globallogic.com>
2 jobs for fix-boot-without-display-connected in 9 minutes and 24 seconds (queued for 1 second)