drm/panel: ilitek-ili9881c: Turn ILI9881C_COMMAND_INSTR() parameters lowercase

Make all ILI9881C_COMMAND_INSTR() parameters consistently lowercase.
No functional change.

Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://lore.kernel.org/r/20250904205541.186001-1-marek.vasut+renesas@mailbox.org