drm/vc4: Fix ->clock_select setting for the VEC encoder

PV_CONTROL_CLK_SELECT_VEC is actually 2 and not 0. Fix the definition and
rework the vc4_set_crtc_possible_masks() to cover the full range of the
PV_CONTROL_CLK_SELECT field.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Eric Anholt <eric@anholt.net>