drm/i915: Move compute part of __intel_set_mode() to separate function

The first function calls done in that function can still cause changes
to the atomic state and may fail. This should eventually be part of our
atomic check function, while the rest of the code in __intel_set_mode()
is the commit hook. So this makes the legacy mode set more atomic-y.

Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>