drm/ast: Inline ast_crtc_dpms() into callers

The function ast_crtc_dpms() is left over from when the ast driver
did not implement atomic modesetting. But DPMS is not supported by
atomic modesetting and the helper is only called to enable or
disable the CRTC sync pulses. Inline the function into its callers.

To disable the CRTC, ast sets (AST_DPMS_VSYNC_OFF | AST_DPMS_HSYNC_OFF)
in VGACRB6. Replace the constants with the correct register constants
for VGACRB6.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240627153638.8765-8-tzimmermann@suse.de