drm/mgag200: Initialize PCI registers early during device setup

So far, PCI option registers were initialized as part of modesetting,
which is late in the process. As these registers control fundamental
operation, they should be set early.

The patch moves the PCI option handling into device register setup,
before even the device MMIO memory is being mapped. No functional
changes made.

Moving the PCI code next to the device-register setup also allows to
remove the has_sdram field from struct mga_device. The state is now
local to the init helper.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200730102844.10995-4-tzimmermann@suse.de