msm: mdss: program PHY_GLBL_TEST_CTRL based on DSI configuration

DSIPHY_GLBL_TEST_CTRL register needs to be programmed based on the PLL
that will be driving the respective PHY which in turn depends on the
configuration of the two DSI devices. Add validity checks to program the
appropriate value to the register.

Change-Id: I294b2d0e95157b32e52e7fa9e1f3de77d9739e1c
Signed-off-by: Padmanabhan Komanduru <pkomandu@codeaurora.org>