drm/amd/display: Rename atomic_commit parameter.

This parameter name is misleading. It's previous
meaning confuses with ASYNC (immidiate flip)
which is totatlly different thing then nonblocking
commit.

Signed-off-by: Andrey Grodzovsky <Andrey.Grodzovsky@amd.com>
Reviewed-by: Harry Wentland <Harry.Wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>