msm: mdss: fix pipe_in_use to check layer extn registers

Previously, only the base mixercfg was used to determine if the pipe
is in use. The extn registers were not checked, resulting in faulty
checks for cursor and any pipe staged in stage6. Fix validation to
include checks for extn registers. And refactor, to get the mixer mask
from utility functions.

Change-Id: Id356fa3255d796abdc596cbfbae6604aa7cf57f0
Signed-off-by: Veera Sundaram Sankaran <veeras@codeaurora.org>