drm/i915/fb: move intel_fb_uses_dpt to intel_fb.c and un-inline

Move fb functions where they belong, and un-inline to avoid looking into
struct drm_i915_private guts in header files.

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/4aa89f113ce6d840d62f50c989e2a1415483557c.1638366969.git.jani.nikula@intel.com