drm/panthor: harden firmware build-info bounds checks

In the Linux kernel, the following vulnerability has been resolved: drm/panthor: harden firmware build-info bounds checks panthor_fw_read_build_info() checks whether the metadata range fits in the firmware image with hdr.meta_start + hdr.meta_size. Both fields are u32, so the addition can wrap and let an out-of-bounds range pass validation. The function also reads the "git_sha: " prefix without first checking that the metadata is long enough, and meta_size == 0 can underflow the NULL terminator index. Use subtraction-based bounds checking and reject metadata that is too short to contain the expected prefix and trailing NULL byte.