drm/i915: Wrappers for display register waits

To reduce the number of explicit dev_priv->uncore calls in the display
code ahead of the introduction of dev_priv->de_uncore, this patch
introduces a wrapper for one of the main usages of it, the register
waits. When we transition to the new uncore, we can just update the
wrapper to point to the appropriate structure.

Since the vast majority of waits are on a set or clear of a bit or mask,
add set & clear flavours of the wrapper to simplify the code.

Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190816012343.36433-7-daniele.ceraolospurio@intel.com