libdrm: fix bad strncpy / snprintf calls

Fix the following bugs reported by IOActive:
* GOOGLE-AN01 - Android libdrm drm_parseDM.c Boundary-CRLF Buffer Overflow
* GOOGLE-AN02 - Android libdrm 'drm_parseDM.c' contentType-CRLF Buffer Overflow
* GOOGLE-AN03 - Android libdrm drm_parseDM.c contentID-CRLF Buffer Overflow
* GOOGLE-AN04 - Android libdrm 'parser_dcf.c' Multiple Headers Buffer Overflow
* GOOGLE-AN05 - Android libdrm 'parser_dcf.c' ContentType Buffer Overflow

Bug: 8727221
Change-Id: If386922e907567f54e26897beffaa1cfe4777896