msm: mdss: Add support xRGB_8888 for FBIOPUT_VSCREENINFO

var->transp.offset is 0 for xRGB format, but currently
code allows only 24, which results in check var failure.
Add this to allow 0 also as transp offset value for xRGB.

CRs-Fixed: 969012
Change-Id: Ie52d9363e4e94bf1e3d1cecb5f391687383042b7
Signed-off-by: Jayant Shekhar <jshekhar@codeaurora.org>