[media] s5p-mfc: Rename IS_MFCV6 macro

The MFC v6 specific code holds good for MFC v7 also as
the v7 version is a superset of v6 and the HW interface
remains more or less similar. This patch renames the macro
IS_MFCV6() to IS_MFCV6_PLUS() so that it can be used
for v7 also.

Signed-off-by: Arun Kumar K <arun.kk@samsung.com>
Signed-off-by: Kamil Debski <k.debski@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>