dm_log_to_buffer logs unconditionally, so calling it directly resulted
in the main message being logged even when the event type isn't enabled
in the event mask.

To fix this, use the new dm_logger_append_va API.

Fixes spurious messages like

 [drm] {1920x1200, 2080x1235@154000Khz}

in dmesg when a mode is set.

v2:
* Use new dm_logger_append_va API, fixes incorrect va_list usage in v1
* Just use and decrease entry.buf_offset to get rid of the trailing
  newline
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>