drm/amd/display: rename function to add otg master for stream

We are renaming acquire first free pipe to add
otg master pipe for stream because the former name
doesn't indicate that it acquires the first free pipe
to use as an otg master pipe. This could cause coding
errors if someone uses it to acquire a different pipe type.

Reviewed-by: Jun Lei <jun.lei@amd.com>
Acked-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Wenjing Liu <wenjing.liu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>