[media] s5p-mfc: Rename IS_MFCV7 macro

Renaming the IS_MFCV7 macro to IS_MFCV7_PLUS for the
addition of MFCv8 support which reuses the v7 code.

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>