drm/i915/psr: CAN_PSR() macro to check for PSR source and sink support.

The global variable dev_priv->psr.sink_support is set if an eDP sink
supports PSR. Use this instead of redoing the check with is_edp_psr().
Combine source and sink support checks into a macro that can be used to
return early from psr_{invalidate, single_frame_update, flush}.

Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180103213824.1405-2-dhinakaran.pandiyan@intel.com