drm/{i915,xe}/fbdev: add intel_fbdev_fb_pitch_align()

drm/{i915,xe}/fbdev: add intel_fbdev_fb_pitch_align()

For reasons still unknown, xe appears to require a stride alignment of
XE_PAGE_SIZE, and using 64 leads to sporadic failures. Go back to having
separate stride alignment for i915 and xe, until the issue is root
caused.

v2: Add FIXME comment, reference issue with Link (Ville)

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Jouni Högander <jouni.hogander@intel.com>
Cc: Maarten Lankhorst <maarten@lankhorst.se>
Link: https://gitlab.freedesktop.org/drm/xe/kernel/-/issues/6220
Fixes: 4a36b339a14a ("drm/xe/fbdev: use the same 64-byte stride alignment as i915")
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://lore.kernel.org/r/ae51d1e224048bdc87bf7a56d8f5ebd0fbb6a383.1756931441.git.jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://lore.kernel.org/r/20251022161054.708388-1-jani.nikula@intel.com