drm/vc4: Fix timings for VEC modes

This commit fixes vertical timings of the VEC (composite output) modes
to accurately represent the 525-line ("NTSC") and 625-line ("PAL") ITU-R
standards.

Previous timings were actually defined as 502 and 601 lines, resulting
in non-standard 62.69 Hz and 52 Hz signals being generated,
respectively.

Changes to vc4_crtc.c have also been made, to make the PixelValve
vertical timings accurately correspond to the DRM modeline in interlaced
modes. The resulting VERTA/VERTB register values have been verified
against the reference values set by the Raspberry Pi firmware.

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