drm/omap: Open code phys_to_page()

phys_to_page() is not available on most architectures.
Just open code it like msm does. Allows COMPILE_TEST=y
builds of omapdrm on other architectures.

Cc: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240408170426.9285-15-ville.syrjala@linux.intel.com
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>