glamor: Use glamor_egl_create_textured_pixmap_from_gbm_bo when possible

Inspired by the modesetting driver.

Fixes screen pixmap corruption with Xorg < 1.20, and as a bonus,
simplifies the code slightly.

Bugzilla: https://bugs.freedesktop.org/107385


Reviewed-by: Alex Deucher <alexander.deucher@amd.com>