egl/drm: Fix misused x and y offsets in swrast_get_image()

egl/drm: Fix misused x and y offsets in swrast_get_image()

It fixes misused x and y variables on the calculation of the memory copy regions.

Cc: Giovanni Campagna <gcampagna@src.gnome.org>
Fixes: 8430af5ebe1ee8119e14 "Add support for swrast to the DRM EGL platform"
Signed-off-by: Mun Gwan-gyeong <elongbug@gmail.com>
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
[Eric: use gbm_bo_get_bpp() instead of local function, split clamp patch]
Signed-off-by: Eric Engestrom <eric.engestrom@imgtec.com>