drm/amd/display: drop unnecessary NULL checks in debugfs

pipe_ctx pointer cannot be NULL when getting the address of
an element of the pipe_ctx array. Moreover, the MAX_PIPES is
defined as 6, so pipe_ctx is not NULL after the loop either.

Detected using the static analysis tool - Svace.

Signed-off-by: Alexey Kodanev <aleksei.kodanev@bell-sw.com>
Signed-off-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>