drm/i915: Introduce better global state handling

Our current global state handling is pretty ad-hoc. Let's try to
make it better by imitating the standard drm core private object
approach.

The reason why we don't want to directly use the private objects
is locking; Each private object has its own lock so if we
introduce any global private objects we get serialized by that
single lock across all pipes. The global state apporoach instead
uses a read/write lock type of approach where each individual
crtc lock counts as a read lock, and grabbing all the crtc locks
allows one write access.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200120174728.21095-15-ville.syrjala@linux.intel.com
Reviewed-by: Imre Deak <imre.deak@intel.com>