drm/i915: introduce INTEL_DISPLAY_ENABLED()

Prepare for making a distinction between not having display and having
disabled display. Add INTEL_DISPLAY_ENABLED() and use it where
HAS_DISPLAY() is used after intel_device_info_runtime_init(). This is
initially duplication, as disabling display still leads to ->pipe_mask =
0 and HAS_DISPLAY() being false.

Note that ever since i915.display_disable was introduced, it has not
affected PCH detection even if it uses HAS_DISPLAY(), as display disable
happens after that.

Since INTEL_DISPLAY_ENABLED() will not make sense unless HAS_DISPLAY()
is true, include a warning for catching misuses making decisions on
INTEL_DISPLAY_ENABLED() when HAS_DISPLAY() is false.

v2: Remove INTEL_DISPLAY_ENABLED() check from intel_detect_pch() (Chris)

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: José Roberto de Souza <jose.souza@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Acked-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190913100407.30991-1-jani.nikula@intel.com