drm: Add DRM_DEBUG_ATOMIC

Atomic state handling adds a lot of indirection and complexity between
simple updates and drivers. For easier debugging the diagnostic output
is therefore rather chatty. Which is great for tracking down atomic
issues, but really annoying otherwise.

Add a new DRM_DEBUG_ATOMIC to be able to filter this out.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Rob Clark <robdclark@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>