drm: move drm_mode related functions into drm_modes.c

Makes more sense and gives better grouping in the DocBook function
reference sections. To make this possible we need to expose two
functions from drm_crtc.c though. To avoid further namespace pollution
in the system wide headers create a new internal header for such drm
internal symbols.

I expect that longer-term we'll add tons more, but since my goal here
is to polish the kerneldoc that's for another day.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>