drm/i915/display: Handle lost primary_port across suspend

icl-dsi is dying on suspend/resume at

	RIP: 0010:icl_update_active_dpll+0x2c/0xa0 [i915]

which appears due to the loss of the time primary_port across suspend.
Protect against the potential NULL dereference by assuming
ICL_PORT_DPLL_DEFAULT unless the port is actively specified otherwise.

Fixes: 24a7bfe0 ("drm/i915: Keep the TypeC port mode fixed when the port is active")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190702140950.7069-1-chris@chris-wilson.co.uk