drm/i915/gem: Convert SPDX headers to single-line format

Replace multi-line SPDX license headers with single-line
equivalents (// SPDX-License-Identifier: MIT or /* ... */ for
headers), as preferred by current kernel coding style.

Signed-off-by: Andi Shyti <andi.shyti@linux.intel.com>
Reviewed-by: Nitin Gote <nitin.r.gote@intel.com>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Mikołaj Wasiak <mikolaj.wasiak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250327232629.2939-1-andi.shyti@linux.intel.com