rd: remove redundant checkDQP and add checkDQP for rd 0

checkDQP is called on bestCU, so no need to call it after each encode. For rd0, call checkDQP after residual encode at depth 0.
rd: check DQP before best cu is copied to the pic in rd 0
compress: remove redundant initializations

skipFlags are set to false during initialization.
compress: remove unnecessary code

reco and resi buffers is not used in this function
compress: remove redundant initializations

depth is already initialized in initsubcu
compress: remove unused sbaccoder load
rd: fix error in merge-skip identfication in rd 1,0

Cost was not calculated if the best mode is merge-skip in rd 1,0
rd: move merge-skip check before other modes are checked

in rd 1,0 merge-skip is chosen only based on sa8d threshols. Checking other Inter/inter modes is unnecessary overhead.
compress: clean up