drm/panic: Ensure drm_panic_type is initialized to a valid value

If a user has set an invalid CONFIG_DRM_PANIC_SCREEN value (such as
"qr_code" without CONFIG_DRM_PANIC_SCREEN_QR_CODE=y due to missing or
incorrect Rust dependencies), there is a panic when accessing
/sys/module/drm/parameters/panic_screen:

[ 12.218375] BUG: unable to handle page fault for address: 0000000796dd8818
[ 12.219737] #PF: supervisor read access in kernel mode
...
[ 12.227167] RIP: 0010:drm_panic_type_get+0x1b/0x30

If drm_panic_type_set() does not find a valid drm_panic_type enumeration
in drm_panic_type_map based on the provided value, it does not change
drm_panic_type from the default -1 value, which is not a valid index for
accessing drm_panic_type_map in drm_panic_type_get(), resulting in the
crash.

Validate the value of CONFIG_DRM_PANIC_SCREEN at boot time via the
return value of drm_panic_type_set() in drm_panic_init() and explicitly
fall back to the default of "user" with a message to the user so that
they can adjust their configuration or fix missing dependencies.

[ 0.800697] Unsupported value for CONFIG_DRM_PANIC_SCREEN ('qr_code'), falling back to 'user'...

Fixes: e85e9ccf3f84 ("drm/panic: Report invalid or unsupported panic modes")
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@igalia.com>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Signed-off-by: Tvrtko Ursulin <tursulin@ursulin.net>
Link: https://lore.kernel.org/r/20260105-drm_panic-handle-invalid-drm_panic_screen-v1-1-55228bd4b0f8@kernel.org