drm/simpledrm: Fix return type of simpledrm_simple_display_pipe_mode_valid()

drm/simpledrm: Fix return type of simpledrm_simple_display_pipe_mode_valid()

When booting a kernel compiled with clang's CFI protection
(CONFIG_CFI_CLANG), there is a CFI failure in
drm_simple_kms_crtc_mode_valid() when trying to call
simpledrm_simple_display_pipe_mode_valid() through ->mode_valid():

[    0.322802] CFI failure (target: simpledrm_simple_display_pipe_mode_valid+0x0/0x8):
...
[    0.324928] Call trace:
[    0.324969]  __ubsan_handle_cfi_check_fail+0x58/0x60
[    0.325053]  __cfi_check_fail+0x3c/0x44
[    0.325120]  __cfi_slowpath_diag+0x178/0x200
[    0.325192]  drm_simple_kms_crtc_mode_valid+0x58/0x80
[    0.325279]  __drm_helper_update_and_validate+0x31c/0x464
...

The ->mode_valid() member in 'struct drm_simple_display_pipe_funcs'
expects a return type of 'enum drm_mode_status', not 'int'. Correct it
to fix the CFI failure.

Cc: stable@vger.kernel.org
Fixes: b7390008b818 ("drm: Add simpledrm driver")
Link: https://github.com/ClangBuiltLinux/linux/issues/1647
Reported-by: Tomasz Paweł Gajc <tpgxyz@gmail.com>
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Sami Tolvanen <samitolvanen@google.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220725233629.223223-1-nathan@kernel.org