disp: msm: dsi: Handle GDSC PM in pre/post clk on/off

Currently, power_state variable does not reflect the GDSC
status. Due to this, there are scenarios where ctrl_power_on
is not happening leading to clock failures. This change
updates power_state based on current status of all the
regulators. GDSC enable is moved to pre_clkon and GDSC
disable is moved to post_clkoff.

Change-Id: I6d9508d5dffda0c94bd3b3bd9b5feb4724dc9dc7
Signed-off-by: Ritesh Kumar <riteshk@codeaurora.org>