drm/ast: Always validate H/V sync flags

The ast driver matches DRM display modes against an internal list of
modes supported by the VBIOS. Matching H/V sync flags between modes is
preferred, but optional. If sync flags are not matching, the driver
would program the VBIOS settings to hardware and let the display handle
the difference.

DRM modes are generated from attached displays or standard mode lines.
Therefore differences to the VBIOS modes are not just cosmetical, but
signal possible incompatibility with the display hardware.

Hence make matching H/V sync flags mandatory. If the VBIOS does not
support a certain mode, we should report it as unsupported. Note that
the VBIOS mode tables all appear to refer to standard modes.

(If sync flags really make no difference to the VBIOS, the ast driver
shouldn't match them in the first place.)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250131092257.115596-8-tzimmermann@suse.de