Add support for swrast to the DRM EGL platform

Add support for swrast to the DRM EGL platform

Turn GBM into a swrast loader (providing putimage/getimage backed
by a dumb KMS buffer). This allows to run KMS+DRM GL applications
(such as weston or mutter-wayland) unmodified on cards that don't
have any client side HW acceleration component but that can do
modeset (examples include simpledrm and qxl)

[Emil Velikov]
 - Fix make check.
 - Split dri_open_driver() from dri_load_driver().
 - Don't try to bind the swrast extensions when using dri.
 - Handle swrast->CreateNewScreen() failure.
 - strdup the driver_name, as it's free'd at destruction.
 - s/LIBGL_ALWAYS_SOFTWARE/GBM_ALWAYS_SOFTWARE/
 - Move gbm_dri_bo_map/unmap to gbm_driiint.h.
 - Correct swrast fallback logic.

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>