drm/i915/display: convert dp_to_i915() to a macro

Avoid looking into the guts of struct drm_i915_private in
headers. Again, converting an inline function to a macro is less than
ideal, but avoids having to pull in i915_drv.h just for the to_i915()
part.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/ed6c43455d13c90ebfed442b196625af5e6ede88.1638366969.git.jani.nikula@intel.com