drm: Move the GET_CAP macros next to the corresponding ioctl structure

It's a tiny bit more logical to find the different capabilities you can
use with the GET_CAP ioctl next to the structure rather than putting
them at the end of the file.

v2: Tab align the litterals (David Herrmann)
v3: Make it clearer that DRM_PRIME_CAP_EXPORT/IMPORT are flags of
    DRM_CAP_PRIME.
v4: Rebase on top of latest bits (DRM_CAP_ASYNC_PAGE_FLIP was
    introduced)

Reviewed-by: David Herrmann <dh.herrmann@gmail.com> (for v2)
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Acked-by: Dave Airlie <airlied@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>