drm: mali-dp: fix Lx_CONTROL register fields clobber

When updating the rotation fields, one of the assignments zeroes out the
rest of the register fields, which include settings for chroma siting,
inverse gamma, AMBA AXI caching, and alpha blending.

Signed-off-by: Mihail Atanassov <mihail.atanassov@arm.com>
Signed-off-by: Liviu Dudau <Liviu.Dudau@arm.com>