drm/msm: Move module parameter 'fbdev' to fbdev code

Define the module's parameter 'fbdev' in fbdev code. No other code
uses it. No functional changes, but simplifies the later conversion
to struct drm_client.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/530555/
Link: https://lore.kernel.org/r/20230403124538.8497-7-tzimmermann@suse.de


Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>