drm/amd/display: Properly guard display_mode_vba with DCN2

[Why]

display_mode_vba is for DCN2 and up. When building for upstream (DCN1
enabled only), there will be a build error, since display_mode_vba.c/h
is stripped out.

Note that building DCN1 only with internal dal-dev is still fine, since
display_mode_vba.h is not stripped out internally - only in upstream.
The make directives therefore stll work, and so will any #include's.

[How]

Since subsequent generations require DCN2 enabled anyways, guard the
makefile directive for display_mode_vba.o with DCN2. Guard any includes
with DCN2. In addition, guard the entire contents of display_mode_vba.h
with DCN2, to simulate the file being stripped out in upstream.

A forward declaration for 'struct display_mode_lib' also needs to be
added in display_mode_lib.h. Previously, display_mode_vba.h contained
the forward declaration, and display_mode_lib.h in turn included it.
This won't work if mode_vba.h is stripped out, requring mode_lib.h to do
so itself.

Signed-off-by: Leo Li <sunpeng.li@amd.com>
Reviewed-by: Harry Wentland <Harry.Wentland@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>