Conditionally skip reference frame check

Conditionally skip reference frame check

For regular inter frames, if the distance from GOLDEN_FRAME is larger
than 2 and if the predicted motion vector of LAST_FRAME gives lower
sse than that of GOLDEN_FRAME, skip the GOLDE_FRAME mode checking in
the rate-distortion optimization. It provides about 5% speed-up at
expense of -0.137% and -0.230% performance down for speed 3. Local
experiment results:

pedestrian 1080p 2000 kbps
66712 b/f, 40.908 dB, 113688 ms ->
66768 b/f, 40.911 dB, 108752 ms

blue_sky 1080p 2000 kbps
51054 b/f, 35.894 dB, 70406 ms ->
51051 b/f, 35.891 dB, 67236 ms

old_town_cross 720p 1500 kbps
14412 b/f, 36.252 dB, 60690 ms ->
14431 b/f, 36.249 dB, 57346 ms

Change-Id: Idfcafe7f63da7a4896602fc60bd7093f0f0d82ca