drm/i915: add min freq control to debugfs

This makes for easier benchmarking and testing.  One can set a fixed
frequency by setting min and max to the same value.

v2: fix whitespace & comment (Eugeni)

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Reviewed-by: Eugeni Dodonov <eugeni@dodonov.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>