media: atomisp: Remove isp->need_gfx_throttle field

Remove the isp->need_gfx_throttle field it is only ever set and
never read.

Also the code setting it is broken, comparing run_mode->val to
ATOMISP_SUBDEV_PAD_SOURCE_VIDEO which are not of the same type /
not part of the same enum.

Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>