drm: avoid warning in drm_load_edid_firmware()

Use "const char *" instead of "char *" in order to avoid this warning:

  drivers/gpu/drm/drm_edid_load.c: In function ‘drm_load_edid_firmware’:
  drivers/gpu/drm/drm_edid_load.c:245:25: warning: initialization discards ‘const’ qualifier from pointer target type [enabled by default]

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>