drm/mediatek: mtk_dpi: Add checks for reg_h_fre_con existence

drm/mediatek: mtk_dpi: Add checks for reg_h_fre_con existence

In preparation for adding support for newer DPI instances which
do support direct-pin but do not have any H_FRE_CON register,
like the one found in MT8195 and MT8188, add a branch to check
if the reg_h_fre_con variable was declared in the mtk_dpi_conf
structure for the probed SoC DPI version.

As a note, this is useful specifically only for cases in which
the support_direct_pin variable is true, so mt8195-dpintf is
not affected by any issue.

Reviewed-by: CK Hu <ck.hu@mediatek.com>
Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://patchwork.kernel.org/project/dri-devel/patch/20250217154836.108895-6-angelogioacchino.delregno@collabora.com/
Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>