drm/msm/mdp5: Reset CTL blend registers before configuring them

Assigning LMs dynamically to CRTCs results in REG_MDP5_CTL_LAYER_REGs
and REG_MDP5_CTL_LAYER_EXT_REGs maintaining old values for a LM that
isn't used by our CTL instance anymore.

Clear the ctl's CTL_LAYER_REG and CTL_LAYER_EXT_REGs for all LM
instances. The ones that need to be configured are configured later
in this func.

Signed-off-by: Archit Taneja <architt@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>