modetest: Allocate dumb buffers with the correct bpp

The modetest application uses libkms to allocate dumb buffers, leading
to overallocation due to the hardcoded 32 bpp value. This can even cause
failures in drivers when the resulting pitch is too large for the
hardware to handle and gets rejected by the driver when creating the
frame buffer.

Fix this by computing the required bpp value and allocating dumb buffers
directly without going through libkms.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>