package/mesa3d: radeonSI: specify drm platform

package/mesa3d: radeonSI: specify drm platform

When BR2_PACKAGE_MESA3D_OPENGL_EGL is selected with RadeonSI drvier,
mesa platform must be specified with drm.

Fixes:
hecking for RADEON... yes
checking for AMDGPU... yes
configure: error: radeonsi requires one of these:
                  1) --with-platforms=drm (X, Wayland, offscreen rendering based on DRM)
                  2) --with-platforms=surfaceless (offscreen only)
                  3) --with-platforms=android (Android only)
                  Recommended options: drm,x11

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>