Split up igt_kms.c/igt_kms.h

lib/igt_kms.* contains both a low-level modeset library, with thin convenience wrappers around core kernel code and libdrm. These functions usually have a drmtest_ prefix (but not all of them).

The other part is a higher-level library around the igt_display and related structures. Those usually come with an igt_ prefix.

The task would be to split this up, and where necessary, fix up the prefixes to match the level a function operates at.