drm/doc: document that PRIME import/export is always supported

Since commit 6b85aa6 ("drm: Enable PRIME import/export for all
drivers"), import/export is always supported. Document this so that
user-space knows what to expect.

Signed-off-by: Simon Ser <contact@emersion.fr>
Reviewed-by: Jeffrey Hugo <quic_jhugo@quicinc.com>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Jeffrey Hugo <quic_jhugo@quicinc.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20230712183156.191445-1-contact@emersion.fr

include/uapi/drm/drm.h

@@ -673,6 +673,9 @@ struct drm_gem_open {
  * Bitfield of supported PRIME sharing capabilities. See &DRM_PRIME_CAP_IMPORT
  * and &DRM_PRIME_CAP_EXPORT.
  *
+ * Starting from kernel version 6.6, both &DRM_PRIME_CAP_IMPORT and
+ * &DRM_PRIME_CAP_EXPORT are always advertised.
+ *
  * PRIME buffers are exposed as dma-buf file descriptors.
  * See :ref:`prime_buffer_sharing`.
  */
@@ -682,13 +685,17 @@ struct drm_gem_open {
  *
  * If this bit is set in &DRM_CAP_PRIME, the driver supports importing PRIME
  * buffers via the &DRM_IOCTL_PRIME_FD_TO_HANDLE ioctl.
+ *
+ * Starting from kernel version 6.6, this bit is always set in &DRM_CAP_PRIME.
  */
 #define  DRM_PRIME_CAP_IMPORT		0x1
 /**
  * DRM_PRIME_CAP_EXPORT
  *
  * If this bit is set in &DRM_CAP_PRIME, the driver supports exporting PRIME
  * buffers via the &DRM_IOCTL_PRIME_HANDLE_TO_FD ioctl.
+ *
+ * Starting from kernel version 6.6, this bit is always set in &DRM_CAP_PRIME.
  */
 #define  DRM_PRIME_CAP_EXPORT		0x2
 /**