drm: move DRM_SWITCH_POWER defines to drm_device.h

Move DRM_SWITCH_POWER out of drmP.h to allow users
to get rid of the drmP include.
Moved to drm_device.h because drm_device.switch_power_state
is the only user.

Converted to enum and added sparse kerneldoc comments.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <maxime.ripard@bootlin.com>
Cc: Sean Paul <sean@poorly.run>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20190108192939.15255-3-sam@ravnborg.org