hevc: calculate the dbf strength in hls_pcm_sample() only if dbf is enabled
hevc_filter: drop redundant checks

ff_hevc_deblocking_boundary_strengths() is never called if the
deblocking filter is disabled for the slice.
hevc_filter: drop more redundant checks

The if() around those loops ensures this condition is always false.
hevc: deobfuscate slice/tile boundary handling for DBF

Use named constants instead of magic numbers, avoid using variables with
inverse meaning from what their name implies.
hevc_filter: move some conditions out of loops
hevc_refs: drop the handling of negative coordinates in ff_hevc_get_ref_list()

It makes no sense and cannot happen in any place from which this
function is called.
hevc_filter: avoid excessive calls to ff_hevc_get_ref_list()

1) each of the loops run within a single CTB, so the relevant reference
list is constant
2) when that CTB is, or lies on the same slice as, the current one, we
can use a simple access instead of a relatively expensive call to
ff_hevc_get_ref_list()