drm/xe: Clarify size of MMIO region

drm/xe: Clarify size of MMIO region

xe_mmio currently has a size parameter that is assigned but never used
anywhere.  The current values assigned appear to be the size of the BAR
region assigned for the tile (both for registers and other purposes such
as the GGTT).  Since the current field isn't being used for anything,
change the assignments to 4MB (the size of the register region on all
current platform) and rename the field to 'regs_size' to more clearly
describe what it represents.  We can use this value in later patches to
help ensure no register accesses accidentally go past the end of the
desired register space (which might not be caught easily if they still
fall within the iomap).

v2:
 - s/regs_length/regs_size/  (Lucas)
 - Clarify kerneldoc description (Lucas)

Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240910234719.3335472-48-matthew.d.roper@intel.com