Replace LOOP_FILTER_BITMASK with a cmake config

This commit replaces the  macro with a cmake config to allow the
feature be easily enabled/disabled using cmake at command line
without modifying code. The feature is defualt disabled, can be
enabled by adding -DCONFIG_LPF_MASK=1 to cmake command, e.g.

cmake -DCONFIG_LPF_MASK=1 ./aom

Change-Id: I592c575949226c98fb60e53d5678e726c723265f
diff --git a/av1/encoder/encoder.c b/av1/encoder/encoder.c
index 397e1c7..27c91de 100644
--- a/av1/encoder/encoder.c
+++ b/av1/encoder/encoder.c
@@ -4041,14 +4041,14 @@
   if (lf->filter_level[0] || lf->filter_level[1]) {
     if (cpi->num_workers > 1)
       av1_loop_filter_frame_mt(&cm->cur_frame->buf, cm, xd, 0, num_planes, 0,
-#if LOOP_FILTER_BITMASK
+#if CONFIG_LPF_MASK
                                0,
 #endif
                                cpi->workers, cpi->num_workers,
                                &cpi->lf_row_sync);
     else
       av1_loop_filter_frame(&cm->cur_frame->buf, cm, xd,
-#if LOOP_FILTER_BITMASK
+#if CONFIG_LPF_MASK
                             0,
 #endif
                             0, num_planes, 0);