For platforms with GMD_ID support (i.e., everything MTL and beyond),
identification of the display IP present should be based on the contents
of the GMD_ID register rather than a PCI devid match.

Note that since GMD_ID readout requires access to the PCI BAR, a slight
change to the driver init sequence is needed --- pci_enable_device() is
now called before i915_driver_create().

v2:
 - Fix use of uninitialized i915 pointer in error path if
   pci_enable_device() fails before the i915 device is created.  (lkp)
 - Use drm_device parameter to intel_display_device_probe.  This goes
   against i915 conventions, but since the primary goal here is to make
   it easy to call this function from other drivers (like Xe) and since
   we don't need anything from the i915 structure, this seems like an
   exception where drm_device is a more natural fit.
v3:
 - Go back do drm_i915_private for intel_display_device_probe.  (Jani)
 - Move forward decl to top of header.  (Jani)
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Andrzej Hajda <andrzej.hajda@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230523195609.73627-6-matthew.d.roper@intel.com