drm/amd/display: Avoid NULL dereference in dc_dmub_srv error paths

In dc_dmub_srv_log_diagnostic_data() and
dc_dmub_srv_enable_dpia_trace().

Both functions check:

  if (!dc_dmub_srv || !dc_dmub_srv->dmub)

and then call DC_LOG_ERROR() inside that block.

DC_LOG_ERROR() uses dc_dmub_srv->ctx internally. So if
dc_dmub_srv is NULL, the logging itself can dereference a
NULL pointer and cause a crash.

Fix this by splitting the checks.

First check if dc_dmub_srv is NULL and return immediately.
Then check dc_dmub_srv->dmub and log the error only when
dc_dmub_srv is valid.

Fixes the below:
../display/dc/dc_dmub_srv.c:962 dc_dmub_srv_log_diagnostic_data() error: we previously assumed 'dc_dmub_srv' could be null (see line 961)
../display/dc/dc_dmub_srv.c:1167 dc_dmub_srv_enable_dpia_trace() error: we previously assumed 'dc_dmub_srv' could be null (see line 1166)

Fixes: 2631ac1a ("drm/amd/display: add DMUB registers to crash dump diagnostic data.")
Fixes: 71ba6b57 ("drm/amd/display: Add interface to enable DPIA trace")
Cc: Roman Li <roman.li@amd.com>
Cc: Alex Hung <alex.hung@amd.com>
Cc: Tom Chung <chiahsuan.chung@amd.com>
Cc: Dan Carpenter <dan.carpenter@linaro.org>
Cc: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Srinivasan Shanmugam <srinivasan.shanmugam@amd.com>
Reviewed-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>