Rename QPixelFormat enum value to avoid conflict with X.h.

This commit fixes a potential compile error where pixelformat.h would
not compile if X.h from x11proto was included first, since X.h #define's
GrayScale as an integer constant.

Task-number: QTBUG-40087
Change-Id: I7208a204259c6d3a13b5d0cbc98a76bb54b8b494
Reviewed-by: Jørgen Lind <jorgen.lind@digia.com>