drm/amd/display: Reorganize DCN30 Makefile

DCN30 has a lot of files in the Makefile, and adding each one next to
the other makes it hard to read and can increase the chance of merge
conflicts. This commit just reorganize the Makefile to put each file
associated with DCN30 in its own line.

Reviewed-by: Chris Park <chris.park@amd.com>
Acked-by: Tom Chung <chiahsuan.chung@amd.com>
Signed-off-by: Rodrigo Siqueira <rodrigo.siqueira@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>