drm/amd/display: Remove redundant checks for res_pool->dccg

The null checks for res_pool->dccg are redundant as it was already
dereferenced previously, as reported by Coverity; therefore the
null checks are removed.

This fixes 6 REVERSE_INULL issues reported by Coverity.

Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Acked-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>