clk: qcom: mdss: fix device crash on continuous splash disabled

TZ introduced a recent change which mandates the scm call
before accessing the pll registers. So, when continuous splash
is disabledin bootloader, the scm call is not made and it causes
device crash when the pll driver tries to access the registers.
This change makes it return with a disabled handoff clk from the
pll driver when continuous splash is disabled, thus not accessing
the pll registers at this point.

Change-Id: Ic487ef733f889d463d149ee347667cd8eb04084f
Signed-off-by: Veera Sundaram Sankaran <veeras@codeaurora.org>