drm/i915: PSR: Increase idle_frames

Idle frames the number of identical frames needed
before panel can enter PSR.

There are some panels that requires up to minimum of 4 idle
frames available on the market. For these cases usually
VBT should be used to configure the number of idle frames,
but unfortunately this isn't always true and VBT isn't being
set at all.

Let's trust VBT when it is set + 1  and use minimum of 4 + 1
when VBT isn't set. "+1" covers the "of-by-one" case.

Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>