drm_hwcomposer: fix crash in hwc_set()

For each display context we need to unconditionally call emplace_back()
on layers_indices to ensure that entries in layers_indices correspond to
the correct display.

BUG=24506167
TEST=still builds and boots ok
Change-Id: I6bc0858471c9ec51615c8ed7d8613529f77cb1ab
1 file changed