drm/amd/display: Remove redundant safeguards for dmub-srv destroy()

[Why]
dc_dmub_srv_destroy() has internal null-check and null assignment.
No need to duplicate them externally.

[How]
Remove redundant safeguards.

Signed-off-by: Lang Yu <Lang.Yu@amd.com>
Signed-off-by: Roman Li <roman.li@amd.com>
Reviewed-by: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>