drm/i915/reset: Reorganize display reset code

drm/i915/reset: Reorganize display reset code

Stop returning the "is there a display?" status from
intel_display_reset_prepare(). I plan to move the pending_fb_pin
into the i915 code, so I need to make that determination already
before intel_display_reset_prepare() is called. Add a new
intel_display_reset_supported() function for that.

v2: Also check display!=NULL for mock tests

Cc: Jouni Högander <jouni.hogander@intel.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patch.msgid.link/20260415210411.24750-4-ville.syrjala@linux.intel.com