If a non-legacy driver calls these it's valid to assume there is
interrupt support. The flag is really only needed for legacy drivers,
which control IRQ enabling/disabling through the DRM_IOCTL_CONTROL
legacy IOCTL.

Also remove all the flag usage from non-legacy drivers.

v2: Review from Emil:
- improve commit message
- I forgot hibmc, fix that

Cc: linux-arm-kernel@lists.infradead.org
Cc: intel-gfx@lists.freedesktop.org
Cc: linux-amlogic@lists.infradead.org
Cc: linux-arm-msm@vger.kernel.org
Cc: freedreno@lists.freedesktop.org
Cc: virtualization@lists.linux-foundation.org
Cc: spice-devel@lists.freedesktop.org
Cc: amd-gfx@lists.freedesktop.org
Cc: linux-renesas-soc@vger.kernel.org
Reviewed-by: NEmil Velikov <emil.velikov@collabora.com>
Reviewed-by: NSam Ravnborg <sam@ravnborg.org>
Signed-off-by: NDaniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190129104248.26607-1-daniel.vetter@ffwll.ch