msm: mdss: expose correct number of supported blending stages

User-space decides how many layers to send for MDP composition using
max blending stage value passed by the driver. Currently driver sends
this value based on its internal enum value which does not reflect the
correct number of blending stages that HW supports. So user-space makes
independent assumption to derive at correct value by deducting 2. This
is incorrect design and may lead to unforeseen issues. Fix it by sending
correct max blending stage value.

Change-Id: Iaaed7b6824e6ef445ca202fb993d1061811b5ce0
Signed-off-by: Ujwal Patel <ujwalp@codeaurora.org>