This speeds up partial updates quite a bit but what is more
important it opens up a way to efficiently implement
lcd_blit_yuv() using hw colorspace conversion.
Tested on rk27generic, hm60x v1 and v2 and on ma9.
Benchmark for hm60x v1 (by mortalis):
HEAD patched
1/1 141fps 138fps
1/4 315fps 395fps
Change-Id: I4cc115786c3139000fc14c49a7290e289cfd6c42