drm/i915/fb: convert parts of intel_fb.c to struct drm_gem_object

drm/i915/fb: convert parts of intel_fb.c to struct drm_gem_object

Prefer the driver agnostic struct drm_gem_object over i915 specific
struct drm_i915_gem_object. Add new intel_bo_* functions as needed.

v2: update comment for intel_bo_is_userptr() (Maarten)

Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Acked-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240918062514.1163290-1-jani.nikula@intel.com