hevc: cleaning up SSE2 and SSSE3 deblocking filters

Signed-off-by: Mickaël Raulet <mraulet@insa-rennes.fr>
hevc: cleanups in SSE2 and SSSE3 loop filters

- use of paddw instead of psllw

Signed-off-by: Mickaël Raulet <mraulet@insa-rennes.fr>
hevcdsp: remove an unneeded variable in the loop filter
beta0 and beta1 will always be the same within a CU

Signed-off-by: Mickaël Raulet <mraulet@insa-rennes.fr>