drm/i915/dmc: handle request_firmware() errors separately

Clarify request_firmware() error handling. Don't proceed to trying to
parse non-existent firmware or check for payload when request_firmware()
failed to begin with. There's no reason to release_firmware() either
when request_firmware() failed.

Also move the message about DMC firmware homepage here, as in other
cases the user probably has some firmware, although its parsing fails
for some reason.

Reviewed-by: Gustavo Sousa <gustavo.sousa@intel.com>
Acked-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/0654bb3480f8d2103225d26f665badead5495532.1713519628.git.jani.nikula@intel.com


Signed-off-by: Jani Nikula <jani.nikula@intel.com>