drm/amd/display: Add dm_logger_append_va API

Same as dm_logger_append, except it takes a va_list instead of a
variable number of arguments. dm_logger_append is now a minimal wrapper
around dm_logger_append_va.

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>