intel: Keep libdrm working without pread/pwrite ioctls

The general direction at this time is to phase out pread/write ioctls and
not support them in future products. The ioctls have already been disabled
in i915 for future products. This means libdrm must handle the absence of
these ioctls. This patch does this by modifying drm_intel_gem_bo_subdata()
and drm_intel_gem_bo_get_subdata() to do the read/write using the
pread/pwrite ioctls first but when these ioctls are unavailable fall back
to doing the read/write using a combination of mmap and memcpy.

A similar solution was added to igt-gpu-tools in commit
ad5eb02eb3 ("lib/ioctl_wrappers: Keep IGT working without pread/pwrite
ioctls").

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Signed-off-by: Ashutosh Dixit <ashutosh.dixit@intel.com>