drm/i915: Add lspcon resume function

As per the software design, we are driving lspcon in
PCON mode. But while resuming from suspend, lspcon can go
in LS mode (which is its default operating mode on power on)

This patch adds a resume function for lspcon, which makes sure
its operating in PCON mode, post resume.

V2: Address review comments from Imre
	- move lspcon_resume call to encoder->reset()
	- use early returns

Signed-off-by: Shashank Sharma <shashank.sharma@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Acked-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1476455212-27893-6-git-send-email-shashank.sharma@intel.com