ARM: dts: msm: disallow use of VDD_GFX CPR retention corner for msm8996

Disallow use of the 400 mV VDD_GFX CPR retention corner since it
has been found that 400 mV is not a sufficiently high retention
voltage across all parts.  Additionally, no power benefits have
been found when using the retention state during graphics
hardware operation.  Also, there is a minor performance penalty
when the retention corner is allowed as software reconfigures
the VDD_GFX supply to the retention voltage during GPU power
collapse and resume before transitioning to an active voltage.

Change-Id: I374d7620373d7fea17818009bec3d171a6072cdc
Signed-off-by: David Collins <collinsd@codeaurora.org>