drm: mali-dp: fix Lx_CONTROL register fields clobber

[ Upstream commit c7ffa59cf03c38b91d6ef01e42c1b52fd7a4f285 ]

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>
Signed-off-by: Sasha Levin <alexander.levin@verizon.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>