drm: mali-dp: Fix transposed horizontal/vertical flip

The horizontal and vertical flip flags were the wrong way around,
causing reflect-x to result in reflect-y being applied and vice-versa.
Fix them.

Fixes: ad49f860 ("drm/arm: Add support for Mali Display Processors")

Signed-off-by: Brian Starkey <brian.starkey@arm.com>
Signed-off-by: Liviu Dudau <Liviu.Dudau@arm.com>