BUG: k_params if seasonal MA

[ChadFulton]

Closes #4789.

[coveralls]

Coverage decreased (-0.09%) to 83.561% when pulling b8a5053 on sarimax-kparams into 68c40b0 on master.

[codecov-io]

Codecov Report

Merging #4790 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #4790   +/-   ##
=======================================
  Coverage   81.11%   81.11%           
=======================================
  Files         581      581           
  Lines       89785    89785           
  Branches    10027    10027           
=======================================
  Hits        72825    72825           
+ Misses      14681    14678    -3     
- Partials     2279     2282    +3

Impacted Files	Coverage Δ
statsmodels/tsa/statespace/sarimax.py	94.17% <ø> (ø)	⬆️
statsmodels/stats/descriptivestats.py	24.13% <0%> (ø)	⬆️
statsmodels/tsa/statespace/mlemodel.py	86.19% <0%> (ø)	⬆️
statsmodels/stats/libqsturng/make_tbls.py	0% <0%> (ø)	⬆️
statsmodels/iolib/summary.py	54.16% <0%> (ø)	⬆️
statsmodels/sandbox/regression/treewalkerclass.py	0% <0%> (ø)	⬆️

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