xf86drm: fix compiler warnings

Used casting to fix warnings about assigning different enum types to
variables. Used error checks in places where snprintf is called and
output might be truncated to fix gcc format-truncation warnings.

v2: Removed a change in drm.h (Simon Ser)
v3, v4: Removed unecessary braces in snprintf (Simon Ser)

Signed-off-by: Eleni Maria Stea <elene.mst@gmail.com>