kernels of the 2.6 variety shoudl have kernel modules with names in the format *.ko. The mtxdrivers-pro-1.1.0_beta-r2 kreates a kernel module with the name mtx.o which should be mtx.ko
Created attachment 44811 [details, diff] A patch to the matrox eclass Does a simple kernel version check and install the correct kernel module.
according to the metadata.xml, this package belongs to x11: <pkgmetadata> <herd>X11</herd> </pkgmetadata>
peper * gentoo-x86/profiles/ (ChangeLog package.mask): Mask x11-drivers/mtxdrivers-pro for removal. WONTFIX.