drm/i915: add display engine uncore helpers

Add convenience helpers for the most common uncore operations with
struct drm_i915_private * as context rather than struct intel_uncore *.

The goal is to replace all instances of I915_READ(),
I915_POSTING_READ(), I915_WRITE(), I915_READ_FW(), and I915_WRITE_FW()
in display/ with these, to finally be able to get rid of the implicit
dev_priv local parameter use.

The idea is that any non-u32 reads or writes are special enough that
they can use the intel_uncore_* functions directly.

v2:
- rename the file intel_de.h
- move intel_de_wait_for_* there too
- also add de fw helpers

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200121113915.9813-1-jani.nikula@intel.com