drm/ast: Add reduced blanking modes for wide screen mode

Signed-off-by: Egbert Eich <eich@suse.com>
Tested-by: Steven You2 Liang <liangyou2@lenovo.com>
Signed-off-by: Y.C. Chen <yc_chen@aspeedtech.com>
v3: based on [PATCH 1/2] drm/ast: Add missing entry to dclk_table[].
    Add reduced blanking modes, improve mode matching to
    identify these modes by thier sync polarities.

[airlied: argh whitespace damage]
Signed-off-by: Dave Airlie <airlied@redhat.com>