drm/ast: astdp: Replace power_on helpers

Replace the helper for controlling power on the physical connector,
ast_dp_power_on_off(), with ast_dp_set_phy_sleep(). The new name
reflects the effect of the operation. Simplify the implementation.
The call now controls sleeping, hence semantics are inversed. Each
'on' becomes an 'off' operation and vice versa.

Do the same for ast_dp_power_is_on() and also align naming of the
register constant with the rest of the code.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240911115347.899148-4-tzimmermann@suse.de