nx-X11/programs/Xserver/hw/nxagent/Display.c: split up nxagentCheckForPixmapFormatsCompatibility() and nxagentInitPixmapFormats().

Don't implicitly call the checking function in the init function (and throw
away the checking functions result...)

Instead, explicitly use the checking function after the init function in other
parts of the code and throw away the checking functions return value
selectively or actually use it while reconnecting.

This is totally different behavior compared to the original one. Hopefully it
doesn't cause any problems (i.e., hopefully there was no good reason for never
using the checking functions return value, but an oversight.)