drm/xe: Separate pure MMIO init from VRAM checkout

We can setup root tile registers mapping at the same time as we
do early mapping of the entire MMIO BAR and keep mandatory VRAM
checkout as a separate step. This will allow us to perform SR-IOV
VF mode detection between those two steps using regular MMIO regs
access functions.

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240327182740.407-2-michal.wajdeczko@intel.com