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

commit ab8df60e3a3b68420d0d4477c5f07c00fbfb078b upstream.

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>
Cc: Amit Pundir <amit.pundir@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>