video: msm: lcdc_s6e63m0: Corrected an unmanaged gamma for autobrightness

Inside the "s6e63m0_set_elvss" method the management of GAMMA_20CD is missing inside the switch block:
case GAMMA_30CD ... GAMMA_100CD
should be
case GAMMA_20CD ... GAMMA_100CD

Have been corrected also some wrong comments.