gpu: ipu-v3: prg: avoid possible array underflow

[ Upstream commit 746d024c3211813946b319411aeb2b47767f8fb0 ]

gcc-8 reports that we access an array with a negative index
in an error case:

drivers/gpu/ipu-v3/ipu-prg.c: In function 'ipu_prg_channel_disable':
drivers/gpu/ipu-v3/ipu-prg.c:252:43: error: array subscript -22 is below array bounds of 'struct ipu_prg_channel[3]' [-Werror=array-bounds]

This moves the range check in front of the first time that
variable gets used.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>