dt-bindings: display: mediatek: update mediatek,dpi.yaml reference

Changeset 9273cf7d ("dt-bindings: display: mediatek: convert the dpi bindings to yaml")
renamed: Documentation/devicetree/bindings/display/mediatek/mediatek,dpi.txt
to: Documentation/devicetree/bindings/display/mediatek/mediatek,dpi.yaml.

Update its cross-reference accordingly.
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/3bf906f39b797d18800abd387187cce71296e5eb.1610535350.git.mchehab+huawei@kernel.orgSigned-off-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/display/mediatek/mediatek,disp.txt

@@ -23,7 +23,7 @@ connected to.
 
 For a description of the display interface sink function blocks, see
 Documentation/devicetree/bindings/display/mediatek/mediatek,dsi.txt and
-Documentation/devicetree/bindings/display/mediatek/mediatek,dpi.txt.
+Documentation/devicetree/bindings/display/mediatek/mediatek,dpi.yaml.
 
 Required properties (all function blocks):
 - compatible: "mediatek,<chip>-disp-<function>", one of