drm/i915/psr2: Fix max resolution supported.

According to spec:
"PSR2 is supported for pipe active sizes up to
3640 pixels wide and 2304 lines tall."

BSpec: 7713

Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180227212913.14083-1-rodrigo.vivi@intel.com