libdrm: random typo fixes

Just some trivial boring typo fixes all over the tree.
READMEs and comments only.

Signed-off-by: Grazvydas Ignotas <notasas@gmail.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>