msm: mdss: fix incorrect mixer configuration on boot

When continuous splash screen is configured, logic for mixer allocation
depending on mdp revision doesn't work because mdp revision is not
properly read until after mixer allocation. At the same time when
configuring the correct mixer, need to make sure that any previous mixer
configuration is properly reset so that only desired mixers are set
correctly in control path.

CRs-Fixed: 462585
Change-Id: I5d5d270e9f2f0c38c990743ae55cd98b2e77bbb0
Signed-off-by: Adrian Salido-Moreno <adrianm@codeaurora.org>