drm/vc4: Fix definition of PAL-M mode

PAL-M is a Brazilian analog TV standard that uses a PAL-style chroma
subcarrier at 3.575611[888111] MHz on top of 525-line (480i60) timings.
This commit makes the driver actually use the proper VEC preset for this
mode instead of just changing PAL subcarrier frequency.

DRM mode constant names have also been changed, as they no longer
correspond to the "NTSC" or "PAL" terms.

Signed-off-by: Mateusz Kwiatkowski <kfyatek+publicgit@gmail.com>